The Northern Ireland man followed up his win in Saturday’s race with another hard-fought success at the famous Dutch TT circuit, with Rea once more locked in battle with Spain’s Alvaro Bautista on the last lap.
He moved into the lead with a pass at Turn 8 and managed to keep Bautista at bay in the dash to the flag.
“I feel really good, especially after a battle like that because the pace was phenomenal,” said Rea.
“I feel like my bike is just as good as the others. As soon as we started to shift third, fourth, fifth, sixth, I was losing a bit on the straight but the mechanical traction I have on the exit was really good.
“When the bike works that well, I have to finish the job.”
Rea was celebrating his 17th win at Assen and his 115th career World Superbike triumph.
Reigning champion Toprak Razgatlioglu set the pace on the Pata Yamaha for much of the reduced nine-lap race until the Turkish rider was pushed back to second by Bautista.
Rea also quickly moved past Razgatlioglu to take up the chase behind the Aruba.it Ducati and pressured Bautista on the final few laps.
There was nothing between the pair as they began the last lap and Rea made a decisive overtake for the lead at Turn 8. From there, the six-time champion put his head down and Bautista was unable to hit back as Rea closed out his second victory of the weekend at round two of the championship.
Razgatlioglu crossed the line in third but the 25-year-old was later confirmed as the runner-up when Bautista – who set a new lap record in 1m 33.620s – was penalised by a drop of one position for exceeding the track limits on the final lap.
Andrea Locatelli (Pata Yamaha), Iker Lecuona (Team HRC) and Loris Baz (Bonovo Action BMW) were the top six.
Going into race two on Sunday (14:15 BST), Rea now leads the standings by seven points from Bautista with Razgatlioglu 20 points further adrift in third.
Eugene Laverty (Bonovo Action BMW) was ruled out of action after injuring his arm in a crash in free practice on Friday.