| 10moSt Johnstone's fourth Tayside derby win of the season edged Dundee closer to the Premiership precipice.
Saints full-back Scott Tanser and centre-forward Callum Hendry put the ineffectual visitors to the sword and completed a campaign whitewash over their troubled neighbours.
Jim McIntyre's Dundee are six points adrift of second-bottom St Mirren and have just four games left to engineer an escape from the automatic relegation spot, having now lost an unprecedented eight top-flight matches in a row.
Tommy Wright's seventh-placed Saints should have won even more comfortably.
The home side were first to threaten in the seventh minute, when wide man Matty Kennedy tested Dundee goalkeeper Seny Dieng at his near post after being set free on the left flank by striker Chris Kane.
Three minutes later Kennedy made space for a teasing cross but Hendry's six-yard diving header flashed narrowly wide.
The same combination almost unlocked the Dundee defence in the 18th minute. This time Hendry's troublesome header was pushed out low at the post by alert goalkeeper Dieng.
The lethargic visitors finally got an effort on target in the 28th minute but Ryan McGowan's header from Nathan Ralph's free-kick was easily held by St Johnstone goalkeeper Zander Clark.
Former Cardiff winger Kennedy was a persistent threat and another delivery saw Kane direct a header straight at the goalkeeper in the 35th minute.
With time running out before the interval Dundee skipper Paul McGowan tried his luck from 20 yards but his effort flashed over the crossbar and Craig Curran drilled a shot straight at Clark.
Six minutes after the restart Kennedy worked space for a shot at the edge of the box but a deflection sent it inches wide.
Dundee centre-back Darren O'Dea was booked for tripping Hendry in the 53rd minute. There was further punishment to be meted out as left-back Tanser stepped up to sweep home the 25-yard free-kick despite Dieng getting a hand to it en route to the top corner.
In the 68th minute former Blackburn youngster Hendry piled on the misery for Dundee when he stooped to head home a Richard Foster cross from close range.
It came minutes after Dundee substitute Andrew Nelson had tugged a rare chance wide. Man of the match Kennedy almost got on the scoresheet but Dieng was able to divert the winger's goal-bound shot.
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