The 2009–10 Euroleague was the 10th season of the professional basketball competition for elite clubs throughout Europe, organised by Euroleague Basketball Company, and it was the 53rd season of the premier competition for European men's clubs overall. The season featured 24 teams from 13 different countries. This season marked the first time in the modern era that a qualifying round was used to determine the last two teams for the regular season. The qualifying round started on September 29, 2009, with the opening game of the proper Euroleague played on October 15, 2010, with a match between Maccabi Electra and Union Olimpija. The season ended at the 2009–10 Euroleague Final Four, which was hosted at the Palais Omnisports de Paris-Bercy in Paris, France, with the semifinals on May 7, and the Final on May 9, 2010.
For the first time in the modern Euroleague era, a preliminary stage was used to determine the last two teams in the regular season. 8 teams competed in qualification rounds, of which the 2 winners advanced to the regular season stage. Those teams joined 22 teams that had qualified directly to the regular season stage.
A maximum of three teams could qualify from any one country through their league position. However, 14 clubs held Euroleague Basketball "A Licenses", giving them automatic spots in the Euroleague Regular Season through 2011–12, regardless of their domestic league finish. These licenses were granted via a formula that considers each team's performance in its domestic league and the Euroleague; the television revenues Euroleague Basketball collects from its home country; and the team's home attendance. The clubs that held A Licenses were:
a Liga ACB was entitled to three Euroleague places by the standard formula. However, because four ACB clubs held A Licences, the league had a minimum of four berths. (The top four places in the 2008–09 ACB season were all occupied by A Licence holders.)
b Lega A was also entitled to three places by the standard formula. However, the 2008–09 season saw only one of the country's two A Licence holders finish in the top three, namely champions Montepaschi Siena. As a result, Lottomatica Roma earned an extra place for Lega A by virtue of its A Licence.
c Lietuvos Rytas was the ULEB Eurocup 2008–09 champion, which carried with it a one-year "C Licence" into the Euroleague Regular Season. However, the club also earned a one-season "B Licence" for the Euroleague by winning its domestic championship, and the league's ranking was sufficiently high to give Rytas direct entry into the Regular Season. As a result, the Eurocup champion's C Licence went to Khimki Moscow Region of the Russian Basketball Super League as the ULEB Eurocup 2008–09 finalist.
Game 1 of each match was played on October 6. Game 2 of the Benetton Treviso-Orléans match was played on October 9, and Game 2 of Maroussi-Alba Berlin was played on October 11. The winners of each match advanced to the Regular Season, with the losers parachuting into the Eurocup.
The survivors from the Regular Season advanced to the Top 16, where they were drawn into four groups of four teams each, playing home-and-home from January 27 through March 11. The draw was held at Euroleague headquarters in Barcelona, starting at 13:00 CET on January 18, and was streamed live on the official Euroleague site.
Key to colors
Top two places in each group advanced to quarterfinals
The Final Four is the last phase of each Euroleague season, and is held over a weekend. The semifinal games are played on Friday evening. Sunday starts with the third-place game, followed by the championship final.