Two tries in quick succession from Oliver Gildart early in the second half proved decisive as Rovers maintained their 100 per cent start to the campaign by ending the Saints' own perfect record.
The Robins dominated for long spells at the Totally Wicked Stadium and, although they briefly fell behind after Saints replied to Mikey Lewis's early try through ex-Wests Tiger and Manly Sea Eagle Curtis Sironen, they thoroughly deserved their success.
Tom Davies put the visitors back ahead and Gildart pressed home the advantage.
Jack Welsby gave the Saints a glimmer of hope in the final quarter but Rovers defended fiercely to secure the points.
After defeat in last year's Grand Final, Rovers are hoping to come of age this term under the guidance of Willie Peters and this was a clear statement of intent.
Rovers were boosted by the return of Man of Steel Lewis from injury and the playmaker got his side off to a superb start.
After a prolonged spell in the Saints' half, Rovers broke through when former North Queensland Cowboy Kyle Feldt failed to claim a high kick ex-Penrith Panther Tyrone May and Lewis scooped up the loose ball to touch down.
Rovers continued to control most of the opening half hour and Lewis almost claimed a second try as Gildart's bullet pass just slipped from his grasp.
It was not until the 31st minute that the Saints finally made an impression and they made it count as Welsby stretched the defence and put a fine kick through for Sironen to race through.
Mark Percival's conversion gave the Saints an advantage which, on the balance of play, they had hardly merited but it was to prove short-lived.
The Robins regained the lead before the break as Davies dived over for a superb try in the right-hand corner after Saints were opened up by fine passes from Elliot Minchella and Jack Broadbent. Lewis added the extras to give Rovers a four-point buffer.
Peters' side moved up a gear after the restart, again profiting from the aerial route as ex-Rabbitoh Joe Burgess outjumped a wrong-footed Feldt to put Gildart in with a rapid offload.
Things got even better for the Robins moments later as Burgess burst clear down the touchline and passed inside for Gildart to race away for another.
The Saints rallied and cut the deficit when the tireless Welsby found his way over in the corner 13 minutes from time, but Rovers' disciplined rearguard was not breached again.
In the other match on Friday, Judah Rimbu's late try heaped more misery on crisis club Salford and handed Castleford their first win of the season with a 22-14 victory at The Jungle.
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