A mistake by goalkeeper Scott Shearer allowed Jordan Bowery to put the Spireites ahead early on but Kayode Odejayi hit back at the start of the second half.
The game was only three minutes old when Shearer came to the left edge of his penalty area but slipped on the wet turf and Drew Talbot crossed for Bowery to fire into an empty net.
The teams had to cope with appalling conditions during a torrential thunderstorm and there could have been further goals with Bowery seeing a 25th minute shot turned behind before Tommy Lee in the home goal saved brilliantly from Ben Pringle in first-half stoppage time.
Rotherham began the second period strongly and they were level in the 51st minute when Odejayi turned away from Sam Hird and drove a right-foot shot past Lee.
It needed a great block by Neal Trotman to prevent Pringle scoring four minutes later but Chesterfield thought they should have had a penalty in stoppage time when Jack Lester's goal-bound shot struck Ian Sharps on the arm.