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 the prior night's 7–2 win over Arizona. He would 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 9-inning game. The 20-year-old ties the record held by Roger Clemens, who performed the feat twice. He also eclipses Bill Gullickson's single-game rookie record of 18 strikeouts in 1980. 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 13 – The Atlanta Braves set an NL record by homering in their 25th straight game, a 10–2 win over the St. Louis Cardinals. This ties the major league mark held by the 1941 Yankees and the 1994 Tigers. The streak will be stopped by the Cardinals the next day.
May 19 – The Cardinals' Mark McGwire hits three home runs in a game for the 2nd 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 any 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 2–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, respectively).
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 4th bases–loaded intentional walk in major league history, and the first since Bill "Swish" Nicholson on July 23, 1944.
June 20 – The Cleveland Indians retire Bob Feller's uniform number 19 prior to the team's 5–3 loss to the Yankees.
June 29 – Uniquely, no major league games are scheduled today: all 30 teams are off.
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 gave up a home run to Moisés Alou on his first delivery in the ninth inning, tying the game. The Padres wound up beating Houston 5-4 in the 10th.
August 10 – At Qualcomm Stadium, the Wendelstedts become the first father and son duo to umpire in the same Major League game. Harry, umpiring in his final Major League season, serves as home plate umpire while his son Hunter, working in his first as a vacation substitute, serves as second base umpire in the Florida Marlins' 3-2 victory over the San Diego Padres.
August 23 – Barry Bonds hits his 400th career home run.
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.
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, to become 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 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 3B, gets one hit in four at bats.
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 St. Louis Cardinals' final game of the season, Mark McGwire hits two home runs against the Montreal Expos for the second straight night, establishing a new MLB record with 70 home runs in a season. Sammy Sosa fails to hit a home run in the Cubs' 4-3 loss to the Houston Astros, leaving him at 66 homers. However, the Cubs loss forces a one-game playoff with the San Francisco Giants for the National League wild card, giving Sosa one final chance to reach McGwire.
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 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.
September 17 – Chet Hoff, 107, pitcher for the New York Highlanders and St. Louis Browns who became the longest-lived major league player
September 30 – Dan Quisenberry, 45, All-Star relief pitcher for the Kansas City Royals who led the AL in saves a record five times and posted the first 40-save season in history; held AL career record from 1987 to 1992 and was Cy Young runnerup twice
October 2 – Gene Autry, 91, owner of the Angels since their formation in 1961 who hoped in vain for the team's first pennant, watching the team fall achingly short three times
October 6 – Mark Belanger, 54, All-Star shortstop and eight-time Gold Glove winner for the Baltimore Orioles, later a players' union official
October 10 – Strick Shofner, 79, third baseman for the 1947 Boston Red Sox
October 14 – Denny Galehouse, 86, pitcher who won 109 games with the Indians, Red Sox and Browns, and Game 1 of 1944 World Series
October 21 – Phil Haugstad, 74, pitcher for the Brooklyn Dodgers and Cincinnati Reds between 1947 and 1952
October 30 – George Schmees, 74, first baseman/outfielder/pitcher for the St. Louis Browns and Boston Red Sox in the 1950s
November 10 – Hal Newhouser, 77, Hall of Fame pitcher for the Detroit Tigers who won back-to-back MVP awards in 1944-45; led AL in wins four times and in ERA and strikeouts twice each; struck out 10 in Game 7 victory in 1945 World Series
November 16 – Russ Meyer, 75, pitcher who won over 90 games for the Cubs, Phillies and Dodgers, known as the "Mad Monk" for his fiery temper
November 20 – Dick Sisler, 78, All-Star first baseman and left fielder for three NL teams whose closing day home run brought the Phillies the 1950 pennant
November 23 – Bob Betts, 70, public announcer at Milwaukee County Stadium for 23 seasons