Jack Payne dinked a shot over goalkeeper Daniel Bentley to put Southend ahead before veteran forward Grant Holt drilled home an equaliser.
Joe Rafferty's first goal of the season put Dale in front and Jamie Allen made it 3-1 with a long-range drive.
Joe Bunney added a fourth to move Rochdale to within two points of the play-off places.
Phil Brown's Southend slip down to 12th, four points adrift of the top six.
Rochdale manager Keith Hill told BBC Radio Manchester:
Media playback is not supported on this device
"We've reached the point of safety when, 10 games ago, we were looking at getting involved in a relegation fight.
"I was really pleased with the equaliser and then the second half.
"The second-half performance just showed our intent."

It was Englishman Fleetwood's first win since 2013, while Scotland's Jamieson took his maiden title in 2012.
The "desert swing" continues at the Qatar Masters on Thursday and Jamieson is one of eight Scots competing.
"It's not always going to be the big names that are winning everything," he told BBC Scotland.
"You're always going to have the marquee players, but there are a number of other guys who can go and win tournaments.
"Four rounds is a long stretch to play good golf for and you've got to manage your game well. If you can keep it together on the day, you're not swinging it as well and still throw in a low round, you're always in with a chance."
Jamieson, 33, finished joint-36th in Abu Dhabi, which was the best placing from the seven Scots in action there.
That came a month after his tied-fourth finish at the Alfred Dunhill Championship in Leopard Creek, South Africa, meaning his 2017 season has got off to an encouraging start.
The end of the 2016 European Tour season was a strain for Jamieson, who narrowly kept his card by finishing 107th in the Race to Dubai.
"My main goal is to knuckle down early in the year so I can push for bigger and better things as the year goes on," said Jamieson, whose European Tour title came at the Nelson Mandela Championship four years ago.
"The last two years for me have been a bit of a struggle compared to my first few years on tour.
"Until the last two years I had never experienced that pressure before and it was pretty uncomfortable - it's not something I would like to endure every year."
The Qatar Masters is held at Doha Golf Club and the tournament has been running since 1998.
It is a par-72 course measuring 7,348 yards, with the par-five ninth hole the longest at 639 yards.
South Africa's Branden Grace successfully defended his Qatar Masters title in 2016 to make it two consecutive wins at the tournament, but has opted not to play there this season.
Scotland's Paul Lawrie claimed the title in 1999 and 2012, with countryman Andrew Coltart winning the inaugural Qatar Masters in 1998.
Yes. Eight of them, including Jamieson and Lawrie.
East Kilbride's Marc Warren will aim to build on his promising opening round in Abu Dhabi last week, as will Aberdeen's Richie Ramsay.
Cockburnspath's David Drysdale also continues his steady start to the season in Doha, with Linlithgow's Stephen Gallacher aiming to improve on last week by making the cut this time around.
Clydebank's Scott Henry is back on the European Tour for 2017 and is in the Qatar Masters field, as is Grantown on Spey's Duncan Stewart.
Germany's two-time major champion Martin Kaymer plays in Doha for the first time since 2014 and in-form world number 10 Alex Noren, from Sweden, also features.
US Open champion of 2010 - Northern Ireland's Graeme McDowell - begins his 2017 European Tour season in Doha.
The winner of the Qatar Masters gets a cheque for just under £335,000.
Rochdale leapfrogged Southend to climb up to eighth place in League One with a comfortable win over the Shrimpers.
[NEXT_CONCEPT]
Scott Jamieson has been heartened by Tommy Fleetwood's triumph at last weekend's Abu Dhabi Championship as he bids for a second European Tour title.