A strong start from the Scottish side saw them set up camp in the Welsh half but a resilient Welsh defence kept them just outside the 22.
Still in possession, the Thistles’ Eilidh Sinclair made the clean break to edge her side forward, but the Welsh defence held firm to win the hosts the penalty and clear back up field.
Taking the line-out, a huge drive from the Welsh pack won a penalty to take play into their opponents half but the Scottish side secured the turnover to take back hold of possession.
A dominant first half from the visitors kept Wales locked in their own half but a huge effort from the Welsh pack denied their visitors a shot at the line.
Eventually the Scottish side spotted the gap to send Ellis Martin over for the opening score. Captain Meryl Smith converting to put the side 7-0 ahead at the break.
Wales came out of the blocks in the second half firing to conjure up an attack in the Thistle’s 22. Taking their chances and playing their way through the phases put the forwards over the whitewash only to be called back for a penalty to deny them their first score.
Going again, Wales looked for the charge once more but a loose pass was nicely spied by Thistles lock, Eva Donaldson, who made the break to race from her own 22 into the Welsh half. Good support from her winger Holly McIntyre saw her go over the line for the Thistles second try. Smith converting to keep the scoreboard in their favour 14-0.
With momentum in their favour, the Thistles went again and a powerful carry in the Welsh 22 put them in for their third try though the hands of Emma Orr. Smith slotting the extras to extend their lead.
The Welsh side were dealt a blow with a yellow card for Jenna DeVera putting them down to 14.
Despite being on the back foot the Welsh side kept digging in deep and almost looked to be in for the try only to be held up over the line.
But the home side kept in the fight and went again, this time making no mistake to put Abbie Fleming in for the try. Niamh Terry converting to put the scores at 7-21 at the final whistle.
WRU Development XV coach Mike Hill said: “This game highlights a few errors that we have got to get right. The set piece had been working really well for us, that was probably the first time it faltered, without that it is hard to get into the game, possession and territory goes off the back of that.
“Some of the intent, individually, was outstanding, I would love to know some of the GPS scores there, the work-rate from the backs in particular was great.
“I think overall, credit to the Thistles. Physically they put us under a lot of pressure.”
