The Whalers played attacking league and opened up the Tasman defence in the game at Trafalgar Park but mistakes proved costly.
"We had our chances on attack but we dropped too many balls," coach David Reedy said.
"We also gave away too many penalties."
The lack of discipline gave opportunities to the Titans and allowed Tasman centre Callum Smith to dominate the game by turning defence into attack.
Smith is the top try-scorer in the league this year and he displayed his magic touch with a superb try.
Smith, who played for the Otago Whalers last year, was the difference between the two teams with his ability to break the line.
The Tasman Titans were also strong in the forwards and made ground by attacking up the middle of the ruck.
The Whalers won only one game in the league but are a young team and most of the squad will be back again next year.
The make-up of the final is in dispute and the finalist and the venue will not be known until later this week.
As it stands at the moment, Tasman would play Southland, at Nelson. But Canterbury has lodged an appeal to the league board and, if successful, it will host the final against Southland, in Christchurch.
The scores
Tasman 40
Otago Whalers 22 (Simon Fuatavai, Joel Tamiora, Shane Wanhi, Drew Elyn tries; Ben McKerchar 3 conversions).
