Reading looked like they had stolen the points when Mathieu Manset scored in the 89th minute - only for Bellamy to lash in from 25 yards.
The Royals went ahead when midfielder Mikele Leigertwood scored for the second time in three days but McDermott then saw top scorer Shane Long limp off with a hip injury.
Dave Jones' side were much improved after the break and grabbed an equaliser when Jay Bothroyd slotted in his 17th goal of the season after an error from Adam Federici.
Manset - a January signing from League Two strugglers Hereford - came off the bench to fire in but Brian McDermott's side were denied a vital three points in their quest for promotion by Bellamy.
Cardiff started well and Peter Whittingham had two efforts from the edge of the area over the top before Bellamy rattled the frame of the goal.
Jem Karacan fouled Bothroyd 25 yards from goal and the on-loan Manchester City man stepped up to whip a superb free-kick over the wall and past Federici only to see the ball thump off the crossbar.
Long was then replaced by Noel Hunt after struggling for a few minutes but Reading grew into the game without him.
First winger Jobi McAnuff tricked his way into the area before thumping a shot straight at City goalkeeper Tom Heaton and Leigertwood then found the opener in the 21st minute.
Jimmy Kebe twisted inside left-back Lee Naylor before cutting the ball back to the edge of the area.
Leigertwood arrived late to drill in a low shot which took a huge deflection off defender Paul Quinn to wrong foot Heaton and find the corner.
Bellamy tested Federici with a low shot from the edge of the area as Cardiff looked to respond before Federici reacted well to claw a header off the line.
Kebe then saw Heaton keep out his powerful drive as the half came to a close.
Michael Chopra came off the bench at half-time and his side were level within three minutes of the restart.
Federici completely mishit a drop-kick to the feet of Bothroyd some 30 yards from goal and the England cap strode into the area, rounded the Australian goalkeeper and passed the ball into the net.
Chopra was looking lively after his introduction and dragged a shot wide on the turn as the hosts pushed on for a winner.
And they came desperately close when Mark Hudson floated a header onto the crossbar and Chopra saw his follow-up bravely blocked by Ian Harte.
Harte then flashed a free-kick across the face of goal before Cardiff substitute Chris Burke wasted a great chance.
Bellamy beat his man and thumped in a low shot which Federici beat away to the penalty spot. Burke was arriving quickly and could not adjust his feet quickly enough as he slapped a shot over the bar.
Manset then grabbed his first Championship goal when Hunt crossed from the right for a tap in but there was still time for an equaliser.
The Bluebirds won a debatable free-kick 30 yards from goal and Bellamy curled a fine effort into the bottom corner.