Jon Rahm's gonna have to wash his mouth out with soap ... 'cause the golf star was caught letting an expletive fly after a putt didn't go his way at the Open Championship on Friday -- passionately yelling out, "F***!!!"
The outburst happened on hole 10 in the second round as Rahm was looking to make par ... but things didn't go as planned, and he let everyone hear his frustration.
"Apologies for any language there," the announcer said shortly after the f-bomb.
The 28-year-old Spanish pro hasn't been the happiest of campers at St.