The Cameroon forward controlled a Thiago Motta pass, spun around and drove home a diagonal strike just after the break to cancel out Mirko Vucinic's first-half goal.
The champions now have 29 points from 12 games, five more than second-placed Juventus, who won 5-2 at Atalanta on Saturday. Inter started the weekend with a seven-point lead.
Inter coach Jose Mourinho had said former title challengers Roma were dangerous despite being in the lower half of the table and missing injured players such as captain Francesco Totti.
He was proved right in the 13th minute when Montenegro striker Vucinic put the visitors in front with a looping header to make amends after his dithering had allowed Inter's defence to recover when he was clean through soon after kick-off.
Inter had keeper Julio Cesar to thank for keeping them on level terms after Eto'o's goal when he produced a fine save to repel a powerful Jeremy Menez shot after the hour mark.
Milan are two points behind Juve after their centre back Thiago Silva scored for both sides in a victory at the Stadio Olimpico against Lazio.
The Brazilian nodded in an Andrea Pirlo free kick from the left to put Milan ahead in the 22nd minute with his maiden Serie A goal.
His compatriot Alexandre Pato also used his head to put away a Ronaldinho cross and make it 2-0 10 minutes before the break before Silva unluckily deflected a Mauro Zarate shot into his own net in the 65th minute to set up a nervy climax.
The visitors held out under pressure to continue their recovery from a shaky start.
Lazio coach Davide Ballardini is now in serious danger of losing his job. His side have not won in the league since August and were hammered 4-1 at Villarreal in the Europa League on Thursday.
Milan leapfrogged Sampdoria, who lost 2-0 at Cagliari after having Marius Stankevicius sent off for a foul at the end of the first half.
Samp share fourth place with Fiorentina after the Tuscan team won 1-0 at Udinese thanks to Juan Vargas's free kick six minutes from time.
