Ewelina Kamczyk opened the scoring just before halftime in the 45th minute; Paulina Tomasiak added a second in the 82nd and sealed the win with a penalty in the 90th, Polish state news agency PAP reported.
The match in the Baltic city of Gdańsk on Tuesday attracted a crowd of more than 10,000, officials said.
Poland remain unbeaten in their last 10 matches, with four wins last year and five victories and a draw so far this year.
"Our women footballers are not stopping! Another convincing win," said Cezary Kulesza, head of the Polish Football Association (PZPN), the national governing body for the sport, after the game.
He added that the crowd marked a record attendance for a women's home match.
With the victory, Poland finished atop League B Group 1 with 16 points and clinched a return to League A, after being relegated last year.
Before the match, officials honoured team captain Ewa Pajor with a commemorative jersey celebrating her 100th appearance for the national team, which she reached in last Friday's game against Northern Ireland.
Poland are ranked 28th in the world by football's global governing body FIFA, while Romania sit at No. 48.
Next month, manager Nina Patalon’s squad will join 15 other teams from across Europe for the Women's Euro 2025 tournament in Switzerland.
Poland head coach Nina Patalon. Photo: PAP/Adam Warżawa
Poland have been drawn into a group with Germany, Sweden and Denmark, and will open their campaign against Germany on July 4.
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Source: IAR, PAP, pzpn.pl