Match ends, RSC Anderlecht 2, Tottenham Hotspur 1.
Tottenham slipped to third in Europa League Group J with their first loss of the campaign at Anderlecht.
Spurs took an early lead when Christian Eriksen capitalised on a mix-up in the hosts' defence.
But Guillaume Gillet rifled in an equaliser before Anderlecht top scorer Stefano Okaka secured victory late on.
Tottenham striker Harry Kane, a second-half substitute, failed to beat goalkeeper Silvio Proto after being played clean through at 1-1.
Both teams were committed to attack in an entertaining match but Spurs paid for wasting early chances, with Erik Lamela the most guilty.
Anderlecht, third in the Belgian Jupiler League, kept Spurs goalkeeper Hugo Lloris busy throughout and were good value for their first victory in six European games.
The sides meet again at White Hart Lane on 5 November with only four points separating the four teams in the group after Monaco beat Qarabag 1-0 to go top.
Spurs pay for profligacy
Tottenham, who have drawn their previous three games in all competitions, are still to reach full throttle this season.
In a frantic opening, they should have taken advantage of a ragged Anderlecht defence and could have been 3-0 ahead within 10 minutes as Eriksen and Lamela shot straight at Proto before the opening goal.
The home side settled after Gillet found the net superbly, and Okaka twice had efforts saved by Lloris.
Although Tottenham's midfield took charge of the first half, their control slipped in the second period and the introduction of Anderlecht substitute Frank Acheampong turned the game in their favour.
His cross set up Okaka for his seventh goal of the season, and it could have been worse for Spurs had Dennis Praet not fluffed a late opportunity.
Spurs' struggles up front continue
With Tottenham suffering from several injuries and Kane picking up a knock in the previous Premier League game against Liverpool, Clinton Njie was handed a full debut up front.
The 22-year-old Cameroon international, who had not scored in seven substitute appearances following his reported £10m arrival from Lyon in August, did not look like opening his Spurs account in Brussels.
Spurs chose to break from midfield rather than use Njie's pace, and he was booked in the 55th minute before being replaced by Kane four minutes later.
The England international at least threatened the Anderlecht goal, but he has now scored only one goal in 13 appearances this season.
Spurs boss angry after defeat
Tottenham boss Mauricio Pochettino: "I am angry, frustrated, disappointed. I don't know which word to describe me.
"We started the game very well. After 10 minutes we scored, we created two or three chances more.
"We lost our focus and after 10 minutes the game changed completely. This was our problem and we need to learn.
"We are young but if you are the best team on the pitch after 10 minutes and you believe you can win the game you need to win the game. We lost our focus."
Anderlecht boss Besnik Hasi: "It's nice to win against such a team. Especially after the poor start against Monaco and Qarabag - we knew it was our last chance today.
"Tottenham clearly came to win and gave the necessary space away for us to exploit."
Stats you need to know
- Tottenham have won just one Europa League game in their last eight away from home (D3 L4).
- Spurs have conceded in nine of their last 10 Europa League matches.
- Anderlecht have beaten an English side in European competition for the first time since October 2000.
Tottenham travel to Bournemouth on Sunday in the Premier League and host Aston Villa on 2 November before facing Anderlecht again three days' later.
- 10PraetBooked at 58minsSubstituted forConteat 90+2'minutes
- 31TielemansBooked at 61mins
- 99OkakaSubstituted forSyllaat 88'minutes
- 93EzekielSubstituted forAcheampongat 66'minutes
- 19DembéléSubstituted forAlliat 65'minutes
- 17TownsendSubstituted forOnomahat 80'minutes
- 14N'JieBooked at 55minsSubstituted forKaneat 59'minutes
- Pol van Boekel
- Shots on Target
Second Half ends, RSC Anderlecht 2, Tottenham Hotspur 1.
Attempt missed. Ibrahima Conte (RSC Anderlecht) right footed shot from the centre of the box misses to the right. Assisted by Steven Defour with a cross.
Substitution, RSC Anderlecht. Ibrahima Conte replaces Dennis Praet.
Foul by Harry Kane (Tottenham Hotspur).
Guillaume Gillet (RSC Anderlecht) wins a free kick in the defensive half.
Attempt missed. Kieran Trippier (Tottenham Hotspur) right footed shot from outside the box is high and wide to the left.
Attempt saved. Dennis Praet (RSC Anderlecht) left footed shot from the centre of the box is saved in the bottom left corner.
Foul by Kieran Trippier (Tottenham Hotspur).
Idrissa Sylla (RSC Anderlecht) wins a free kick on the left wing.
Substitution, RSC Anderlecht. Idrissa Sylla replaces Stefano Okaka.
Eric Dier (Tottenham Hotspur) wins a free kick in the defensive half.
Foul by Guillaume Gillet (RSC Anderlecht).
Foul by Erik Lamela (Tottenham Hotspur).
Frank Acheampong (RSC Anderlecht) wins a free kick on the left wing.
Attempt missed. Dennis Praet (RSC Anderlecht) right footed shot from the right side of the box is too high. Assisted by Steven Defour.
Offside, Tottenham Hotspur. Dele Alli tries a through ball, but Harry Kane is caught offside.
Substitution, Tottenham Hotspur. Josh Onomah replaces Andros Townsend.
Attempt missed. Leander Dendoncker (RSC Anderlecht) header from the left side of the six yard box is close, but misses to the left. Assisted by Kara with a headed pass following a corner.
Corner, RSC Anderlecht. Conceded by Jan Vertonghen.