Poland took control of the match after Romania were reduced to 10 men in the 22nd minute when Radu Drăgușin was sent off.
The hosts capitalised on the numerical advantage two minutes later when Lewandowski converted from the penalty spot to put Poland ahead.
Lewandowski doubled Poland's lead shortly after the restart, scoring in the 47th minute to put the hosts firmly in control.
Romania's misery deepened in the 49th minute when Virgil Ghiță turned the ball into his own net, giving Poland a 3-0 advantage.
Piotr Zieliński added a fourth in the 62nd minute before Jan Bednarek made it 5-0 four minutes later.
Lewandowski completed his hat-trick in the 86th minute, rounding off a dominant display from the Poland captain.
Poland controlled 67 per cent of possession and registered 24 shots, including nine on target, while Romania managed 10 attempts and only three efforts on target.
The victory gives Poland seven points from their opening three Group B4 matches, following a goalless draw with Bosnia and Herzegovina and a 3-1 defeat by Sweden. Romania remain on one point after three matches.
Poland will travel to Bosnia and Herzegovina for their next Nations League fixture on Monday, October 5.
Match details
• Poland: Robert Lewandowski 24' (pen), 47', 86', Virgil Ghiță 49' (og), Piotr Zieliński 62', Jan Bednarek 66'
• Romania: No goals
• Venue: PGE Narodowy, Warsaw
• Competition: UEFA Nations League, League B, Group B4
• Date: 2 October 2026