The Pole claimed his first title of the year by recovering after Wattimena had reeled off five successive legs to turn a 4-1 deficit into a 6-4 lead.
Ratajski responded by winning the final four legs, taking advantage of missed doubles before sealing victory on double 10.
“I’m really happy. I don’t know what to say,” Ratajski said after clinching the title.
“It was a close game. Jermaine played very well and I’m really happy that I survived it."
“I didn’t expect it. I would have been happy with the last eight or last four, so to win the title is more than I expected," Ratajski continued.
“I will do my best in Blackpool (World Matchplay). I hope I will play well and hopefully I can win that first match. We will see.”
Ratajski's run to the title included victories over Gian van Veen, who he'll face again in Blackpool next week, as well as defending champion Nathan Aspinall and World Cup teammate Sebastian Bialecki.
Wattimena, meanwhile, reached his first-ever European Tour final after an impressive weekend that included emphatic wins over Kim Huybrechts and Ryan Joyce, a deciding-leg victory against Luke Woodhouse and a 7-3 semi-final triumph over Damon Heta.
“I’m very happy with that,” Wattimena said. “Thank you very much to the crowd. It was a magic day.
“I missed some chances, but Krzysztof was 4-1 up and he missed some chances as well, so I think it’s a fair result. Krzysztof was the better player, so well done to Krzysztof.”
Results
Round three
Jermaine Wattimena 6-1 Ryan Joyce
Luke Woodhouse 6-5 Jonny Clayton
Dirk van Duijvenbode 6-3 Wessel Nijman
Damon Heta 6-4 Michael van Gerwen
Sebastian Bialecki 6-4 Kevin Doets
James Wade 6-2 Jan Schmidt
Nathan Aspinall 6-4 Ross Smith
Krzysztof Ratajski 6-3 Gian van Veen
Quarter-finals
Jermaine Wattimena 6-5 Luke Woodhouse
Damon Heta 6-2 Dirk van Duijvenbode
Sebastian Bialecki 6-2 James Wade
Krzysztof Ratajski 6-4 Nathan Aspinall
Semi-finals
Jermaine Wattimena 7-3 Damon Heta
Krzysztof Ratajski 7-5 Sebastian Bialecki
Final
Krzysztof Ratajski 8-6 Jermaine Wattimena