The striker took his tally to five goals in two games when hitting the opener and winner in an ABAX Stadium thriller.
Marriott fired Posh ahead two minutes into the second half with a low shot that crept in courtesy of a kiss off the inside of the far post.
But Rotherham were soon back on level terms as Kieffer Moore, who like Marriott hit a hat-trick in his previous league appearance, headed in captain Lee Frecklington's left-wing cross.
Marriott then steered a Marcus Maddison cross against a post to be denied by the woodwork for the second time after seeing an early effort hit the underside of the crossbar.
Rotherham thought they had hit the front moments later only to see their celebrations cut short by a raised flag when Jamie Proctor nodded in from close range.
But a cracking contest was settled with 15 minutes to go when Marriott got the better of dithering defender Semi Ajayi and lobbed a glorious finish over stranded Rotherham goalkeeper Richard O'Donnell.
Match report supplied by the Press Association.
Match ends, Peterborough United 2, Rotherham United 1.
Second Half ends, Peterborough United 2, Rotherham United 1.
Corner,  Rotherham United. Conceded by Michael Doughty.
Kieffer Moore (Rotherham United) is shown the yellow card for hand ball.
Corner,  Rotherham United. Conceded by Michael Doughty.
Attempt missed. Joe Newell (Rotherham United) right footed shot from a difficult angle on the left is close, but misses to the left.
Attempt saved. Andrew Hughes (Peterborough United) left footed shot from outside the box is saved in the top left corner.
Joe Mattock (Rotherham United) is shown the yellow card for a bad foul.
Idris Kanu (Peterborough United) wins a free kick in the defensive half.
Foul by Joe Mattock (Rotherham United).
Attempt missed. Jack Marriott (Peterborough United) right footed shot from outside the box misses to the right.
Attempt missed. Kieffer Moore (Rotherham United) right footed shot from outside the box is high and wide to the right.
Joe Newell (Rotherham United) is shown the yellow card.
Substitution, Peterborough United. Andrew Hughes replaces Leonardo Da Silva Lopes.
Substitution, Peterborough United. Chris Forrester replaces Marcus Maddison.
Substitution, Rotherham United. Anthony Forde replaces Joshua Emmanuel.
Anthony Grant (Peterborough United) wins a free kick in the defensive half.
Foul by Lee Frecklington (Rotherham United).
Attempt missed. Lee Frecklington (Rotherham United) header from the right side of the six yard box misses to the right.
Corner,  Rotherham United. Conceded by Anthony Grant.
Foul by Anthony Grant (Peterborough United).
Jon Taylor (Rotherham United) wins a free kick on the left wing.
Corner,  Rotherham United. Conceded by Jonathan Bond.
Attempt saved. Kieffer Moore (Rotherham United) header from the centre of the box is saved in the bottom left corner.
Substitution, Rotherham United. Jon Taylor replaces Ryan Williams.
Goal!  Peterborough United 2, Rotherham United 1. Jack Marriott (Peterborough United) right footed shot from outside the box to the high centre of the goal. Assisted by Jack Baldwin.
Anthony Grant (Peterborough United) wins a free kick in the attacking half.
Foul by Ryan Williams (Rotherham United).
Leonardo Da Silva Lopes (Peterborough United) wins a free kick on the right wing.
Foul by Darren Potter (Rotherham United).
Substitution, Rotherham United. David Ball replaces Jamie Proctor.
Substitution, Peterborough United. Idris Kanu replaces Junior Morias.
Attempt missed. Joe Newell (Rotherham United) left footed shot from the right side of the box is close, but misses to the left.
Corner,  Peterborough United. Conceded by Jamie Proctor.
Junior Morias (Peterborough United) wins a free kick on the right wing.
Foul by Joe Mattock (Rotherham United).
Gwion Edwards (Peterborough United) wins a free kick on the right wing.
Foul by Darren Potter (Rotherham United).
Foul by Steven Taylor (Peterborough United).
Kieffer Moore (Rotherham United) wins a free kick in the defensive half.
Jack Marriott hit a second-half double as Peterborough continued their perfect League One start by beating Rotherham.