Disappointing results including a 0-0 draw at home to fellow high-flyers Lyon last week had seen Marcelo Bielsa's men slip to third in the standings, but Sunday's routine win keeps them in the hunt at the top.
After a goalless first half, Bielsa introduced Batshuayi from the bench at the interval and the Belgian opened the scoring within a minute of coming on, bundling Dimitri Payet's low cross over the line.
Alaixys Ramao doubled their lead in the 67th minute, before Andre Ayew added the third a few moments later, tapping in from Benjamin Mendy's square pass.
Batshuayi netted his sixth goal in five games right at the end, pouncing on a rebound after Ayew danced past multiple defenders and saw his initial effort parried by Rudy Riou.
In-form Monaco remain hot on Marseille's tail, however, as a comfortable 3-1 victory at Reims kept Leonardo Jardim's men just four points behind.
Jardim was without a host of players – including Dimitar Berbatov and Danijel Subasic – following Monaco's away-goals defeat of Arsenal in the UEFA Champions League, but Fabinho's emphatic fifth-minute opener sent them well on their way to victory.
Anthony Martial doubled the Champions League quarter-finalists' tally nine minutes later, lobbing Reims goalkeeper Kossi Agassa with a clever finish and, although Diego pulled one back in the second half, Nabil Dirar restored the two-goal cushion 11 minutes from time.
In the day's early kick-off, a 52nd-minute red card for Lille's Manchester City-owned attacker Rony Lopes gave Saint-Etienne the edge, before the hosts ultimately went on to win 2-0.
Lopes, 19, produced a crude challenge on Franck Tabanou and received a straight red card, before Max Gradel netted twice in the final half-hour to keep Saint-Etienne four points behind Monaco.
