Saints had led 19-14 at the break, but had no response to the home side in the second half, as the Rhinos ran in four tries after the interval to go joint top of the table.
Saints had a dream start when youngster Steve Tyrer scored a try after just 2 minutes at Headingley, Paul Scultorpe converting. Leeds hit back with tries from Scott Donald and Danny McGuire, before James Roby's try tied things back up at 12-12.
Kevin Sinfield slotted over a penalty to edge the Rhinos in front, but Saints went in ahead at half time after Paul Wellens' try was converted by Steve Tyrer, and Sean Long knocked over a one pointer on the hooter for the second week running.
But it was Clinton Toopi who set the pattern for the second 40, his try in the first minute of the second half setting the Rhinos on their way to a fantastic win. Jamie Peacock, Brent Webb and Rob Burrow added scores to give a much needed win over the champions, in front of a mammoth 21,975 at Headingley.