The visitors, put in to bat on Wednesday, amassed 6-348 off their 50 overs to win by 99 runs on the Duckworth–Lewis–Stern (DLS) method after rain interrupted Namibia's reply and they finished on 8-162 off 31 overs, chasing a reduced target of 262.
Hermann, one of three debutants in a weakened South African side, hit 20 boundaries, including three sixes, in his 126-ball knock, putting on 190 for the second wicket with Tony de Zorzi, who made 72 off 84 balls
He survived a run-out on 30 after an umpire's error was not rectified because no television reviews were available, and his knock equalled the record set by teammate Matthew Breetzke last year, who also scored 150 on his ODI debut against New Zealand in a tri-series in Pakistan.
Hermann raced from 100 to 150 in just 32 balls before being caught at cover off Ruben Trumpelmann, who was the pick of the home bowlers in claiming 4-68.
Namibia were always behind the required run-rate and their cause was made even more arduous when their innings was twice interrupted by rain. When they returned to the crease after the second delay, their target was reset to a near-impossible 167 runs off eight overs.
Wednesday's clash was the first ODI between the neighbours, who along with Zimbabwe co-host next year's World Cup. They meet again at the Namibia Cricket Ground on Friday.
