Jansen was bothered by an injury during spring training and was slow-played after his 2017 workload, resulting in two blown saves during a shaky April. He turned it around in May, June and July and was chosen for his third straight all-star game in 2018.
He was effective against Atlanta and Milwaukee in the postseason, not allowing a run, but again struggled in the 2018 World Series, blowing two save opportunities as he allowed game-tying home runs to Jackie Bradley Jr. in Game 3 (which went 18 innings) and Steve Pearce in Game 4 as the Dodgers lost the series to the Boston Red Sox in five games.
Jansen suffered a recurrence of his irregular heartbeat prior to a game in Denver on August 9, 2018. He was placed on the 10-day disabled list and the team reported he would likely miss a month of the season. Doctors determined the condition could be controlled with medication until the offseason, so Jansen returned to the hill just 11 days later. However, the heart medication he took, which included beta blockers, had the side effects of leaving him feeling fatigued and unemotional. He gave up four home runs in his first two outings back, blowing both save opportunities, and after he protested the effects of his medication his doctors switched him to aspirin. Jansen stayed in Los Angeles when the Dodgers returned to Denver in September because he was at high risk of a stroke if he returned to elevation. Jansen completed the season and provided scoreless relief in seven of nine postseason appearances, allowing two runs in 10.2 innings (1.69 ERA).
On November 26, 2018, Jansen underwent a 5-1/2 hour heart ablation procedure, again performed by Dr. Koonlawee Nademanee, in Los Angeles. After the procedure the doctor told Jansen that small veins near the site of his 2012 procedure had grown and were agitated by dehydration, which caused the irregular signals that occurred in Denver. Jansen dedicated himself to a healthier diet after recovering from the operation and reported to spring training 25 pounds lighter. He no longer has to take blood-thinning medication.
During the 2018 MLB Players Weekend, Jansen wore the nickname "KENLEYFORNIA" on his jersey, a portmanteau blending "Kenley" and "California" in a reference to Jansen's entrance song, "California Love" by 2Pac and Dr. Dre.