The animal, believed to be an adult porpoise, was spotted swimming in shallow waters near Seafield.
Jodie Barker, who went down to the shore with her two children to see the porpoise up close, became concerned that it was at risk of becoming beached and alerted the SSPCA.
But fortunately the tide came in and the porpoise was able to happily swim away.
Porpoises are rarely seen this far into the Solway Firth, although local haafnetters saw a school while out fishing last month.
