Britt Assombalonga has now scored 13 goals in the Championship this season.
Middlesbrough came from behind to keep their play-off hopes alive while Reading's safety was confirmed despite defeat.
Teenager Danny Loader bent in from 20 yards early on for his first senior goal to give the Royals an early lead.
But Boro, who hit the woodwork twice either side of Loader's strike, responded when Lewis Wing netted and Britt Assombalonga's penalty, after a foul on George Saville, proved to be the winner.
Reading, though, were able to officially celebrate survival after Rotherham lost at West Bromwich Albion.
It would have taken an unlikely goal-difference swing for Jose Gomes' side to have been in trouble, even if Rotherham had won their final two games, but they can now start planning for life in the Championship next season.
Middlesbrough, meanwhile, briefly looked as though they were surrendering the final play-off spot to Derby when they trailed to England Under-19 international Loader's opener.
They remain in contention for a top-six place after Wing and Assombalonga turned the match around - but could do with a favour from Swansea City, who host the Rams on Wednesday.
Middlesbrough manager Tony Pulis told BBC Tees:
"We played some fantastic football, especially on the right-hand side. I thought [Jonny] Howson, [Paddy] McNair and [Lewis] Wing were absolutely fantastic down that side.
"I was really pleased for Wingy. We kept it as quiet as we possibly could- we didn't want anyone to know he was going to be fit. I apologised to him because I kept him on for 90 minutes, and I shouldn't have done. He's been the find of the season. He's got a good character, a good attitude and he wants to improve. That's what will take him further.
"If Derby win their two games, they win their two games, but we've kept the pressure on them, and that's what we wanted to do today."
Reading manager Jose Gomes told BBC Radio Berkshire:
"We deserve it. It's the result of the work of a lot of people, from the owner of the club to the players and the technical staff. Now, I feel everyone is working like a family and everybody wants to help the first team. In the beginning, I didn't feel this.
"A special word to our fans, who were suffering and suffering and waiting for good results but never gave up. A lot of the points we got were the result of their support.
"From the beginning, I really believed we would achieve our target. [With] the way that the players work and the way they accept ideas, it was only a matter of time. Hopefully the last game on Sunday will be like a beautiful party."
