Guernsey finished the season on 82 points, one above Trojans
A one-point defeat by Trojans on Saturday was still enough to see Guernsey Rugby Club finish top of the London Two South West league table.
The 16-15 defeat at Footes Lane meant Guernsey finished one point ahead of their rivals and will play in London One South next season.
Trojans, from Southampton, will go into play-offs for a promotion into the same division.
Guernsey captain Blair Campbell described the victory as "bittersweet".
He said: "We got promotion, that was our goal, but we lost the game so there are mixed feelings at the moment.
Guernsey rugby club win the London Two South West title despite losing their final match of the season to their nearest rivals Trojans.
"I think we really dug it out - there's chances we missed but all in all I'm proud of the guys.
"We'll assess in the coming weeks for the Siam Cup, but I think if you reflect on the season it was quite successful."
Coach Jordan Reynolds said: "We've been working really hard for five years for this - we're bitter at losing but Trojans were quite good.
"They really needed that win to get the play-off spot and they got it, so fair play to them."
Trojan's captain Chris Hamp said: "It's a bit of an empty win if I'm honest, we came over here trying to get the five points and stop them getting anything.
"It was going to be a big ask anyway, but it's nice to get the win, which is the main thing - it was a cracking game."