Goals from Jason Demetriou and Sam Mantom early in the second half turned the contest on its head after the Millers had gone into the break with a 1-0 lead.
Rotherham took the lead on the half-hour mark when Rob Kiernan scrambled the ball into his own net from a corner.
Four minutes into the second half Chiedozie Ogbene was penalised for a handball inside the area and Demetriou stepped up to score the penalty.
Moments later, Mantom brought down a Elvis Bwomono pass and calmly slotted home to put the home side in front.
But Southend's lead last for just eight minutes as Ladapo latched onto Clark Robertson’s cross to level proceedings.
And that is how it stayed as the Millers remained in the top six.
Elsewhere in League One, Donervon Daniels conceded his third penalty in four matches as Doncaster Rovers threw away a one-goal lead to lose 2-1 at AFC Wimbledon.
Rovers dominated the first half and took a deserved lead into the break through Kwame Thomas’ header.
But they failed to match their efforts in the second half and were picked off by the renewed hosts.
Daniels jumped into a challenge he was never likely to win, giving Marcus Forss the perfect opportunity to level from the spot.
And Callum Reilly completed the turnaround with a breakaway goal 20 minutes from time.
The defeat means that Doncaster haven't won a league game since their 7-1 thumping of Southend on October 22.
In League Two, Bradford City moved up to fifth place despite playing out a goalless draw at Leyton Orient.
Gary Bowyer's charges are now three games unbeaten as Matt Palmer and Shay McCartan returned from injury.
Conor Wilkinson’s header struck the crossbar on 11 minutes and that was the closest either side game in an evenly-matched contest.