The stream running through Singleton Park, Swansea, turned a bright green.
Local resident Tom Judge was on a run with his girlfriend at the park at about 5pm on August 3 when they spotted the strange sight.
"It was bright florescent green and didn't look natural at all. We were both really shocked," said 23-year-old Mr Judge.
"We took some pictures because we didn't think people would believe how green it really was. The whole stream was green, but it seemed worse near the sea.
"It didn't just wash away - it was green for the whole 25 minutes we were there. We run there three or four times a week and have never seen anything like it."
It's understood the river is now back to its normal colour
Natural Resources Wales (NRW) is investigating the cause of the colour change on Monday, but so far it remains a mystery.
A spokesman for NRW said: "Natural Resources Wales has been made aware of green water in a stream at Singleton Park in Swansea and we are investigating the issue.
"We would urge anyone who sees possible pollution or issues in rivers and streams to report it immediately to our incident line on 0300 065 3000. The sooner we know about it, the sooner we can investigate."
