After Doncaster went home from Church Bank last week with the win, few gave Llandovery a chance of reversing that defeat but they came through with a last minute try from their diminutive centre Jack Roberts.
The Drovers also gained a valuable bonus point for scoring four tries and top Pool 8 with three wins from four games as a result. The Premiership side now host Melrose and travel to Nottingham in the New Year and remain favourites to make the knockout stages.
Despite the big Doncaster pack providing a physical threat, the Drovers got more ball than in the game at Church Bank but trailed 22-18 at the break.
Two tries from scrum half Chris Hallam and a penalty try had bettered tries from centre Adam Warren and hooker Luke Lewis.
Flanker Lloyd Phillips and Doncaster hooker Rhys Buckley swapped tries but in a grandstand finish, Roberts tore through for the winner.