Jacob deGrom is really back.

The Mets ace, who hasn’t pitched in a big-league game in 391 days, was impressive in his five-inning start Tuesday night in the Amazin’s 5-1 loss to the host Nationals.

The Mets played it safe with their returning ace, who allowed one run in five frames, taking him out of the game after 59 pitches, 46 of which were strikes. He also struck out six.

DeGrom opened with a 1-2-3 first by striking out Victor Robles and Luis Garcia and then getting Yadiel Hernandez on a grounder to short. He also hit 102 mph during the impressive inning.

DeGrom also pitched a scoreless second inning. He gave up his first hit in the second inning, a single to Keibert Ruiz, who was thrown out trying to stretch a single into a double. He struck out two more batters (Nelson Cruz and Joey Meneses) and had four strikeouts through two frames.

The only run he allowed was in the fourth when he gave up and RBI double to Luis Garcia, who drove in Victor Robles who singled earlier in the inning.

It was a long way back for deGrom. The right-hander — who owned a 1.08 ERA in 15 starts and was on track for a third Cy Young award in four years last season — never returned after the All-Star break because of forearm and elbow discomfort.

In spring training this year, deGrom appeared in two games before he was shut down from throwing with a stress reaction on his right scapula.