Barcelona will again face seven-time winner AC Milan in the Champions League, and Bayern Munich starts the defense of its title in a group that includes Manchester City.
Thursday's draw in Monaco put Barcelona in Group H with Milan, Ajax and Celtic — four teams who have been European champions 16 times between them.
Barcelona and Milan were grouped together in 2011-12 and then met in the quarterfinals, before facing each other in the round of 16 last season. Barcelona won three of the six encounters and drew two of them.
"You never get easy groups in the Champions League," Barcelona forward Lionel Messi said. "Of course we know Milan well and we also played Celtic last season, while Ajax have a lot of European history. So we're looking forward to some very tough, very exciting, very beautiful games."
Bayern, which beat Borussia Dortmund 2-1 in last year's final at Wembley Stadium, is in Group D along with City, CSKA Moscow and Viktoria Plzen.
Dortmund is in one of the toughest groups of the tournament alongside Arsenal, Marseille and Napoli.
Real Madrid will face Juventus in another blockbuster pairing, with Galatasaray and Copenhagen joining them in Group B. Madrid is bidding for a record 10th title and lost to Dortmund in the semifinal last season, while Juventus, which lost to Bayern in the quarterfinals, has gone from strength to strength under Antonio Conte and won a second straight league title last campaign.
New Manchester United manager David Moyes will get his first taste of the Champions League group stage after taking over from Alex Ferguson this summer, and the Premier League champions have been put in Group A along with Shakhtar Donetsk, Bayer Leverkusen and Real Sociedad.
Chelsea won the Champions League in 2012 but went out at the group stage last year before going on to capture the Europa League. Now managed again by Jose Mourinho and back in Europe's top competition, the team should fare better in a group consisting of Schalke, Basel and Steaua Bucharest.
Group C is made up of Benfica, big-spending Paris Saint-Germain, Olympiakos and Anderlecht, while FC Porto, Atletico Madrid, Zenit St. Petersburg and Austria Vienna are in Group H.
The first group games will take place on Sept. 17 and 18.
Group A_Manchester United (England), Shakhtar Donetsk (Ukraine), Bayer Leverkusen (Germany), Real Sociedad (Spain).
Group B_Real Madrid (Spain), Juventus (Italy), Galatasaray (Turkey), Copenhagen (Denmark).
Group C_Benfica (Portugal), Paris Saint-Germain (France), Olympiakos (Greece), Anderlecht (Belgium).
Group D_Bayern Munich (Germany), CSKA Moscow (Russia), Manchester City (England), Viktoria Plzen (Czech Republic).
Group E_Chelsea (England), Schalke (Germany), Basel (Switzerland), Steaua Bucharest (Romania).
Group F_Arsenal (England), Marseille (France), Borussia Dortmund (Germany), Napoli (Italy).
Group G_FC Porto (Portugal), Atletico Madrid (Spain), Zenit St. Petersburg (Russia), Austria Vienna (Austria).
Group H_Barcelona (Spain), AC Milan (Italy), Ajax (Netherlands), Celtic (Scotland).