The overtime bill rose by £4.9m over the year with policing of the Nato summit being cited as the reason.
Overall, the four Welsh forces spent £16.9m in 2014-15, up from almost £12m in 2013-14. The highest overtime payment was to a Gwent Police speed camera operator who earned £23,024.
BBC Radio 5 Live obtained the figures  using Freedom of Information requests.
Thirty nine forces responded to the request and of those, South Wales Police reported the biggest percentage increase for overtime spending in the last year, from £5.2m in 2013/14 to £8.4m in 2014/15.
A force spokesman said 1,500 of its officers were among the 9,500 from across the UK involved in looking after the security of world leaders last September.
The extra cost was recovered from the Home Office.
The force said further overtime costs were due to special events, such as rugby and football matches, which could also be recovered from event organisers.
Meanwhile Gwent Police, which was involved with policing the Nato events at the Celtic Manor, saw its overtime costs increase by 40% to a bill of £2.7m, up from £1.9m.
Dyfed-Powys Police saw overtime costs increase by 33% to a total of £2.4m, up £600,000, and North Wales Police spent an additional £350,000, a rise of 12%, to £3.2m compared to 2013-14.

Callum Gathercole, 20, from Surrey began the 3,000-mile (4,828km) Atlantic Challenge race in the Canary Islands on 20 December.
He arrived in Antigua's English Harbour in the Caribbean early on Wednesday.
Mr Gathercole, who is studying aerospace engineering at the University of Bristol, set a time of 58 days, 15 hours and 15 minutes.
His achievement as the youngest solo trans-oceanic rower has been confirmed by the Ocean Rowing Society, the official adjudicator of ocean rowing records for Guinness World Records.
Mr Gathercole experienced "some difficult days rowing in tough conditions" as he battled tropical storms, 40ft (12m) waves and sleep deprivation.
"It was a brilliant challenge that I thoroughly enjoyed taking on," he said.
His challenge has raised more than Â£85,000 so far for the Princess Alice Hospice in Esher, Surrey, in memory of his father.
He has also represented Great Britain in the European and World Internationals in 2012 and 2013.
The previous youngest solo ocean rower was 22-year-old Katie Spotz from the USA who rowed the Atlantic east-to-west between 3 January and 14 March 2010.
The previous youngest male solo ocean rower was the UK's Tommy Tippetts, 22, who rowed the same route as Ms Spotz between 21 January and 12 April 2012.
Mr Gathercole's success comes nine days after that of two other University of Bristol students - Freddie Wright and Jack Galsworthy - both 21, who have become the youngest pair to row the Atlantic.
The men were at sea for 47 days, 14 hours and 46 minutes before crossing the finishing line in their 6.9m boat 'Blue Steel'.
Mr Wright described the race as "an awesome experience" and something "we would definitely do again".
Nearly £17m was paid out as overtime to police officers and staff across Wales last year.
[NEXT_CONCEPT]
A British student has become the youngest person to row solo across an ocean.