The operative phrase here is ‘in professional golf’. According to Guinness World Records, the lowest, officially recognised, score for 18 holes in the history of golf was the 55 recorded by Australian professional Rhein Gibson at River Oaks Golf Club in Edmond, Oklahoma on May 12, 2012. Gibson, 26, signed for twelve birdies and two eagles in his 16-under-par total on the 6,698-yard par-71 course. However, while he was a member of the Golfweek National Pro Tour – a developmental feeder tour to the PGA Tour – at the time, his ‘unbeatable’ record, while substantiated by witnesses, was set in a non-competitive round.
In competitive professional golf, the record low score for 18 holes was achieved on the Alps Tour – a developmental, third level tour, behind the European Tour and Challenge Tour – in 2019. On September 5 that year, 23-year-old Irish professional David Carey carded eleven birdies, including seven in his first nine holes, in the opening round of the Cervino Open at Cervino Golf Club in Breuil-Cervinia, Italy. His 18-hole total, an 11-under-par 57, on the 5,801-yard, par-68 course gave him a four-shot lead at the end of the first day and, after subsequent back-to-back rounds of 67, he went on to win the 54-hole tournament in a playoff with local player Edoardo Lipparelli.
Prior to September 5, 2019, the record low score for 18 holes was 58. That score was first achieved by Ryo Ishikawa in the final round of The Crowns, on the Japan Golf Tour, in 2010, but subsequently matched, twice, in 2016. That year, German Stephan Jaeger shot a 12-under-par 58 in the first round of the Ellie Mae Classic on the Web.com Tour, now the Korn Ferry Tour, and American Jim Furyk did likewise in the final round of the Travelers Championship on the PGA Tour.