Watsonians remain in the hunt for a top-four finish after taking the honours in a fiercely contested derby at Myreside.
The home side owed their success to a massive tackle count, particularly in the second half as Boroughmuir battled to take something from the game and prolong their own slender play-off hopes that had been revived by three successive bonus point wins.
The visitors had the bulk of possession but Watsonians reached the break with a 13-0 lead, with Lee Millar booting a long-range penalty then breaking three tackles to create a try for Josh Rowland which he also converted. Millar then added a second penalty before returning to defensive chores to help Watsonians withstand another flurry of attacks.
Boroughmuir eventually found a way through in 52 minutes when Rory Drummond crashed over following a lineout. And with their hopes of securing a play-off berth dwindling by the minute, the visitors threw everything they had at a resolute home defence.
Then, deep into injury time and with the match already lost, Boroughmuir had one final chance to take a narrow defeat bonus point. However, once again they were cruelly denied when a knock-on left them with nothing to show for their efforts.