The 1998 season was also marked by an expansion to 30 teams (16 in the NL, 14 in the AL), with two new teams–the Arizona Diamondbacks in the National League, and the Tampa Bay Devil Rays in the American League–added to the MLB. To keep the leagues with even numbers of teams while allowing both leagues to have a new team, the Milwaukee Brewers were moved from the American League Central Division to the National League Central Division. The Detroit Tigers were shifted from the American League East to the American League Central, while the Devil Rays were added to the American League East. The Diamondbacks were added to the National League West, making the NL have more teams than the AL for the first time .
The defending World Series champions Florida Marlins finished last in the NL East Division at 54-108, making it the first (and only time) ever that a team went from winning the World Series one year to finishing with 100 or more losses and last in their division the following year.
April 1 – The expansion Tampa Bay Devil Rays win their first game in franchise history, beating the Tigers 11–8. Fred McGriff has four RBI on three hits.
April 2 – By hitting a home run in Colorado's 6–4 win over Arizona at Bank One Ballpark, Rockies outfielder Ellis Burks sets a major league record by having homered in 33 different stadiums.
April 5 – The Arizona Diamondbacks win their first game in franchise history 3–2, over the San Francisco Giants. Andy Benes gets the win for the 1–5 D'backs.
April 10 – The Los Angeles Dodgers' Mike Piazza becomes the fifth NL player in history to hit grand slams in consecutive games by homering in a 7–2 win over the Houston Astros. Piazza also homered with the bags full, while driving in six runs, in last night's 7–2 win over Arizona. He'll hit another on April 24 to tie the major-league record for slams in a month.
May 6 – In one of the finest pitching efforts ever, Chicago Cubs rookie right-hander Kerry Wood fans 20 Houston Astros in a 2–0, one-hit victory to tie the major league mark for strikeouts in a nine-inning game. The 20-year-old ties the record held by Roger Clemens, who performed the feat twice. The only Houston baserunners come from an infield single to Ricky Gutiérrez in the 3rd inning and a hit batter. Wood also becomes the second pitcher in baseball history to record a single-game strikeout total equal to his age (in 1936, 17-year-old Bob Feller struck out 17 batters). Wood strikes out the first five batters of the game, and seven in a row between the 7th and 9th innings, tying Jamie Moyer's Cubs record for most consecutive strikeouts.
May 11 – In a 4–2 win over Arizona, Kerry Wood strikes out 13 Diamondbacks in seven innings. By doing so, Wood sets a major league record with 33 strikeouts over two consecutive games.
May 19 – The Cardinals' Mark McGwire hits three home runs in a game for the second time this season, leading St. Louis to a 10–8 victory over the Philadelphia Phillies. He is only the 12th player in history to have a pair of 3–HR games in the same season. McGwire drives in six of the Cardinal runs as he reaches the 20 home run mark faster than other player in history.
May 25 – Cleveland's David Bell becomes the third player in major league history to play against a team managed by his father. Bell's two–run double brings home the go–ahead run in the Indians 7–4 win over Buddy Bell's Detroit Tigers. Bump Wills and Moisés Alou are the only other players to appear in games against their fathers (Maury Wills and Felipe Alou).
May 28 – With Arizona leading the Giants, 8–6, in the bottom of the 9th with the bases loaded, manager Buck Showalter orders reliever Gregg Olson to intentionally walk Barry Bonds to bring home the Giants' 7th run. It is only the fourth bases–loaded intentional walk in major league history, and the first since Bill "Swish" Nicholson on July 23, 1944.
June 30 – The Chicago Cubs' Sammy Sosa hits his 33rd home run of the season in a game against the Arizona Diamondbacks. Sosa's 20th home run in the month of June is a new MLB record for most home runs in one month.
July 26 – Trevor Hoffman's bid to set a major league record with 42 straight saves ended when the San Diego closer gives up a home run to Moisés Alou on his first delivery in the ninth inning, tying the game. The Padres defeat Houston 5–4 in the 10th, but Hoffman blows his chance at history.
July 31 - The Houston Astros acquire flame-throwing pitcher Randy Johnson of the Seattle Mariners for three minor league players; Johnson caught fire upon arriving in Houston. In 11 regular-season starts with the Astros, he had a 10–1 record, a 1.28 ERA, and 116 strikeouts in 84⅓ innings, and pitched 4 shutouts. Johnson finished seventh in the National League Cy Young voting despite pitching only two months in the league, and helped Houston win their second straight National League Central division title.
August 25 – The Toronto Blue Jays' Roger Clemens strikes out 18 in a 3–0 victory over the Kansas City Royals. He becomes the first pitcher ever to record three games of 18 or more strikeouts. Clemens allows only three hits and does not walk a batter.
August 31 - Chicago's Kerry Wood throws 135 pitches in a game against a mediocre Cincinnati Reds team and wakes up the next morning with a sore right elbow. It is the 9th game of the season that manager Jim Riggleman allowed the 20-year old rookie to thrown more than 120 pitches. The Cubs decided to shut-down the super-star for the remainder of the regular season. Kerry would return for one game in the playoffs, before blowing out his elbow in spring training the next year.
September 4 – The New York Yankees win their 100th game of the season, defeating the Chicago White Sox 11–6. The Yankees reach that mark five days faster than the 1906 Chicago Cubs and 1954 Cleveland Indians.
September 5 – Mark McGwire becomes the third player in major league history to reach 60 home runs, as the St. Louis Cardinals beat the Cincinnati Reds 7–0. McGwire joins Babe Ruth and Roger Maris with 60 home runs in a single season.
September 8 – Mark McGwire breaks Roger Maris' 37-year-old home run record, lining historic No. 62 just over the wall in left field with two outs in the fourth inning. McGwire's solo shot off the Chicago Cubs' Steve Trachsel—among the shortest he would hit all year—sets off a wild celebration at Busch Stadium. The Cubs' Sammy Sosa, who hit his 58th home run earlier in the game, is on the field to congratulate McGwire, creating an iconic image of the 1998 home run race. In the sixth inning of the same game, the Cardinals' J.D. Drew makes his major league debut pinch-hitting for pitcher Kent Mercker.
September 11 – The Florida Marlins lose to the Atlanta Braves 8–2, becoming the first World Series champion in history to lose 100 games the next season.
September 15 – Ken Griffey, Jr. hits homer #52 and drives in the 1,000th run of his career in the Mariners' 12–7 win over the Minnesota Twins. He becomes the fourth–youngest player in history to reach the milestone, after Mel Ott, Jimmie Foxx and Lou Gehrig. A day later, Griffey would collect his 20th stolen base of the season to become just the third player in major league history to record at least 50 homers and 20 steals in the same season; Willie Mays and Brady Anderson are the others.
September 20 – Cal Ripken, Jr. of the Baltimore Orioles takes himself out of the lineup prior to the game with the New York Yankees to end his major league record consecutive game streak at 2,632. The Orioles lose the historic game by a score of 5–4. Ryan Minor, Ripken's replacement at third base, gets one hit in four at bats.
September 23 – At Milwaukee County Stadium, Sammy Sosa hits his 64th and 65th home runs as the Chicago Cubs jump out to a 7–0 lead against the Milwaukee Brewers. However, the Brewers erase the deficit by scoring eight runs in the last three innings, the last three coming when Brant Brown drops a Geoff Jenkins fly ball with two out in the ninth inning; the error allows Mark Loretta, Jeff Cirillo and Jeromy Burnitz to score. The Cubs stay tied with the San Francisco Giants for the wild-card lead when they could have led by one game with three games left. Ironically, the error comes 90 years to the day of Fred Merkle's baserunning mistake, which led to the Cubs ultimately winning the National League pennant and, to date, their last World Series title.
September 25 – Just hours after Sammy Sosa hits his league-leading 66th home run, pulling ahead of Mark McGwire for the first time all season, McGwire hits his 66th in a game against the Montreal Expos.
September 27 – In the San Diego Padres' final regular season game, left fielder Greg Vaughn hits his 50th home run of the season, a career high and a San Diego Padres record for home runs in a season. This marks the first time in major league history that four players – Vaughn (50), Griffey (56), Sosa (66) and McGwire (70) – hit at least 50 home runs in the same season. Also during this game, Trevor Hoffman records his 53rd save of the season, tying the National League record set by the Cubs' Randy Myers in 1993.
September 27 – In recording his first-ever Major League win, a 2–1 decision over the Detroit Tigers at the Skydome, Roy Halladay of the Toronto Blue Jays, a week removed from his Major League debut, has what would have been the second no-hitter in Blue Jay history broken up by a Bobby Higginson home run with two out in the ninth, the only hit he will allow. The no-hitter also would have been the third to be pitched on the final day of a regular season, joining the combination of Vida Blue, Glenn Abbott, Paul Lindblad and Rollie Fingers in 1975 and Mike Witt's perfect game in 1984. The home run ball is caught, ironically, by Dave Stieb, himself a three-time victim of a no-hitter being broken up with two out in the ninth (his last two starts of the 1988 season and a perfect game bid in 1989) before finally pitching the Blue Jays' only no-hitter to date, in 1990.
September 28 – In a one-game playoff, the Chicago Cubs defeat the San Francisco Giants 5–3 to secure the final playoff spot in the National League. For the third game in a row, the Cubs' Sammy Sosa gets two hits, but no home runs, leaving him at 66 home runs for the season, four fewer than Mark McGwire, who pulled ahead of Sosa with five home runs in his final three games.
Tom Glavine of the Atlanta Braves wins his second National League Cy Young Award in an extremely close vote over two San Diego Padres pitchers: Trevor Hoffman and Kevin Brown. Glavine, who receives 11 first-place votes to Hoffman's 13 (Brown receives the remaining 8), becomes the first National League pitcher since the league instituted its four-vote system in 1970 to win the award despite receiving fewer first-place votes than another player. Glavine tallied 99 points (Hoffman – 88, Brown – 76), with 5 points being awarded for each first place vote, 3 for each second-place vote, 2 for third, and 1 for fourth. Another oddity is the fact that Hoffman, Brown, and Rod Beck (who did not receive a single point in the Cy Young Award voting) finished higher than Glavine in the MVP voting, despite Glavine's Braves finishing with the best record in the National League.
^With an odd number of teams (15), only seven games would be able to be scheduled in each league on any given day during the intra-league portion of the regular season. Thus, one team in each league would have had to be idle on any given day. This would have made it difficult for scheduling, in terms of travel days and the need to end the regular season before October. SeeMajor League Baseball#League organization. If each league had wished to remain at fifteen teams, the schedule would have had to include one inter-league game during each day of intra-league play. Instead, with each league now having an even number of teams, interleague games occur only in certain parts of the regular season.