Mourinho's old Tottenham striker Troy Parrott came off the bench to score the winner in Andalucia as the Irishman notched his first goal for the club since joining from AZ Alkmaar last month.
That broke Real Madrid's 100% three-game winning streak from the start of 2026/27 as Kylian Mbappe saw a late penalty saved and Carlos Espi's spectacular equaliser was ruled out for offside by VAR.
Mourinho was in spiky mood in his post-match press conference and claimed Betis were lucky to snatch all three points as his team failed to react in the heat of the battle.
"The Betis players were cleverer than us. They were more more streetwise than the Madrid players, who are very honest.
"With the backing of a fantastic crowd, they managed to gain these kinds of advantages.
"Madrid's players were naïve. Betis players tried to impact the referee, but Madrid's players, when something deserved a yellow card, they got straight back up. When you play teams with this mentality, it's an important factor.
"They played for a draw and got lucky in the end."