Mick McCarthy's Ipswich lost 3-2 to Blackburn on the final day on the Championship but still booked their place in the play-offs as results elsewhere went their way.
Ipswich began the afternoon in fifth place with a three-point cushion to seventh and could hardly have started better at Ewood Park with Daryl Murphy volleying them ahead after just two minutes.
But Jordan Rhodes and Craig Conway struck before half-time to put Blackburn 2-1 up and, with results elsewhere, Ipswich dropped to sixth, hanging on courtesy of goal difference.
Rudy Gestede made it 3-1 from close range in the second half to shred the nerves of the visiting fans, whose side fought frantically to get back in the game.
They hit a second through Murphy's late penalty - his 27th goal of the season - and, although they could not draw level, any worries were alleviated with a loud cheer from the visiting end signalled Derby's capitulation.
Derby only had to claim a point at home to Reading to pip Ipswich to that final play-off spot but suffered a collapse at home to Reading, losing 3-0 to ensure another season in the Championship.
Kwesi Appiah gave Reading an early lead and after Adam Federici saved a Darren Bent penalty, second half goals from Michael Hector and Gareth McCleary left Derby outside the top six.
Ipswich will play arch-rivals Norwich in play-offs, after the Cannaries recorded a 4-2 victory over Fulham at Carrow Road.
Both Norwich and Middlesbrough were already guaranteed a place in the play-offs before today's games. Boro could have theoretically overtaken Norwich for third place but drew 0-0 with Brighton.
Brentford are also through to the play-offs as Alex Pritchard, Jota and Andre Gray were on target to secure a 3-0 win over relegated Wigan, while results elsewhere played right into the Bees' hands.
Having kicked off in seventh place, they needed a little help from their friends, and Reading obliged by beating Derby while former Brentford loanee Jordan Rhodes was on target as Blackburn sank Ipswich.
Therefore the Bees, famously denied automatic promotion from League One by Marcello Trotta's penalty miss two seasons ago, were on the right end of events for a change to secure fifth place and keep their Barclays Premier League dreams alive.
Elsewhere, Bournemouth were crowned champions after beating Charlton 3-0.
Two goals from Matt Ritchie and one from Irish international Harry Arter saw off the mid-table Addicks on their home turf, who were outclassed from the off and were two down within 12 minutes.
Watford will also play in the Barclays Premier League next season but the Hornets were denied the Championship title after a last-gasp Atdhe Nuhiu equaliser earned Sheffield Wednesday a 1-1 draw at Vicarage Road.
Having secured promotion last week following a 2-0 victory at Brighton, and subsequent results involving Middlesbrough and Norwich City going their way, the Hornets knew they simply had to match or better what the Cherries achieved at Charlton Athletic to go up as champions.
One goal from Matej Vydra - his 16th of the season - looked to have been enough but Watford had several other fine opportunities to make what was a comfortable victory more reflective of their dominance against the Owls.
They were opportunities they were to regret not taking because although they were in complete control, they were to be undone when Nuhiu headed home after the Hornets had failed to clear their lines from a late free-kick.