The Polish number one last tasted victory on the PDC ProTour in February 2020, but saved his best display of the year for the 30th Players Championship to scoop the £10,000 title.
Cullen had looked on course to win his third Players Championship of 2021 as he took leads of 3-1 and 7-5 during a gripping final.
Ratajski, though, followed up earlier finishes of 139 and 108 by landing double eight to break throw in leg 13 and then taking out 87 to send the final into a deciding leg.
Against the throw, he was crucially first to a finish and checked out 112 in clinical fashion to complete his comeback.
"It was incredible for me, very emotional," admitted Ratajski afterwards. "I'm really happy with this win, it's very important for me.
"It's a long time since I won the last tournament and it's something incredible for me, really.
"At the start of the season I played good in the stage tournaments but in the floor tournament's I wasn't so good.
"In these last three days I've played really brilliant darts and felt really good. It's a big thing for me."
Ratajski had produced some outstanding darts earlier in the day, including a 107.77 average to defeat Simon Whitlock, a 12-darter in the deciding leg against form star Jonny Clayton and a 100 average in his 7-3 semi-final win over Darius Labanauskas.
Cullen, meanwhile, achieved three ton-plus averages during his run to a fourth Players Championship final of the year.
He came from 3-0 down to win his semi-final against Michael van Gerwen with a stunning average of 111.28, hitting seven doubles from as many attempts and six 180s in a remarkable comeback.
Van Gerwen had been bidding to claim back-to-back ProTour wins after picking up his first ranking title of 2021 on Wednesday.
The world number three had continued his fine form with three ton-plus averages and defeats of Daryl Gurney and Damon Heta in reaching the semi-finals, only to see Cullen's super-show end his hopes.
Lithuania's Labanauskas enjoyed his best run in a Players Championship since May 2019 as he saw off UK Open champion James Wade, Dirk van Duijvenbode and in-form Ryan Searle in five wins to end the ProTour season on a high.
Edgar won through to his first Players Championship quarter-final since February 2019 as he overcame top seed Michael Smith and UK Open finalist Luke Humphries on Thursday.
World Series of Darts Finals champion Jonny Clayton and in-form Dutch ace also reached the last eight alongside Wade and Edgar.
Simon Whitlock's run to the last 16 was enough to secure him a place in the Ladbrokes Players Championship Finals, which features the top 64 players from the final 2021 Players Championship Order of Merit.
2021 Players Championship 30 Results
Last 16 onwards. For all the results and detailed stats from the Super Series, head to Dart Connect (tv.dartconnect.com/events/pdc)
Last 16
- Matthew Edgar 6-5 Luke Humphries
- Michael van Gerwen 6-4 Damon Heta
- Joe Cullen 6-4 Callan Rydz
- Danny Noppert 6-5 Martin Schindler
- James Wade 6-5 Mensur Suljovic
- Darius Labanauskas 6-5 Dirk van Duijvenbode
- Krzysztof Ratajski 6-2 Simon Whitlock
- Jonny Clayton 6-1 Stephen Bunting
Quarter-finals
- Michael van Gerwen 6-3 Matthew Edgar
- Joe Cullen 6-3 Danny Noppert
- Darius Labanauskas 6-4 James Wade
- Krzysztof Ratajski 6-5 Jonny Clayton
Semi-finals
- Joe Cullen 7-4 Michael van Gerwen
- Krzysztof Ratajski 7-3 Darius Labanauskas
Final
- Krzysztof Ratajski 8-7 Joe Cullen
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