The 2018 Rugby Championship saw Barrett miss the first Bledisloe Cup test against Australia's Wallabies, before he re-joined his two brothers in the All Blacks, for the second test against the Wallabies, on 25 August. With Rieko Ioane out injured, Barrett once again started at fullback, with Ben Smith and Waisake Naholo starting on the wings. Barrett's brother Beauden joined Jordie and Scott as a Man of the Match-winner during the 2018 season, scoring four tries in their 40-12 win over the Wallabies, which saw a 67-minute performance from Jordie, before he was replaced by Damian McKenzie.
Barrett missed another test in the Rugby Championship, with his Hurricanes team-mate, Nehe Milner-Skudder, earning a re-call for New Zealand after a long period of time being injured. He was then bought back into the starting lineup, at fullback, for the first test against South Africa at Westpac Stadium, on 15 September 2018. Barrett scored the opening try of the test, only 4 minutes in, but the test quickly went into South Africa's favour. Barrett, as well as his brother Beauden, both performed poorly in the test. Barrett notably let in Aphiwe Dyantyi for South Africa's first try, then throwing a pass to Willie le Roux, for South Africa's second try, only five minutes after Dyantyi's opening try. Barrett was replaced by Damian McKenzie, who also performed poorly, in the 58th minute. The All Blacks suffered a shock 34-36 loss to South Africa and Barrett did not play again during the Rugby Championship.
On the 2018 end-of-year tour, Barrett played in two tests. His first was a poor performance against Japan, which the All Blacks won with a 69-31 margin. For the final test of the 2018 season, against Italy, Barrett was unusually given a start on the right wing, with Waisake Naholo on the left wing and Rieko Ioane being used off the bench. Although he played on the wing, Barrett was the best-performing player on the field against Italy and won his second Man of the Match award for 2018, scoring four tries in the test, becoming only the third All Black of the decade to score four tries in a match, after his brother Beauden, and former All Black, Zac Guildford.