Masters Tournament: Finished runner-up at the Masters, one stroke behind Patrick Reed, for his best result at the event. For the first time in eight starts at Augusta National, recorded four par-or-better rounds (70-72-65-67). Earned his third runner-up in a major (2nd/2018 Masters Tournament, T2/2014 The Open Championship, T2/2014 U.S. Open) and third top-five in the last four (2nd/2018 Masters Tournament, T5/2017 Open Championship, T5/2017 U.S. Open). With third-round 65, was one of just four players to post a bogey-free round during the week (Rory McIlroy/65, Jon Rahm/65, Si Woo Kim/65) with all four occurring in the final round.
On June 3, 2018, DeChambeau won the Memorial Tournament in Dublin, Ohio, in a sudden-death playoff against Kyle Stanley and An Byeong-hun, after the three finished regulation play tied at −15. After Stanley bogeyed the first hole of sudden death, DeChambeau then proceeded to win with a birdie on the second hole, giving him his second win on the tour.
Hyundai Tournament of Champions: Finished T6 in a Monday-start-Tuesday-finish event, the season-opening, 54-hole Hyundai Tournament of Champions. The event "started" twice during the regularly scheduled first round Friday and later Sunday, but both times PGA TOUR rules staff halted play due to unplayable windy conditions and subsequently voided all scores that had taken place. As the first player off the tee, he ended up hitting three "opening" shots of the season.