Reid - on a season-long loan from Championship side Yeovil - swooped for his third goal of the season to power Conor Hourihane's free-kick past Rovers goalkeeper Steve Mildenhall from close range.
In a derby devoid of first-half action, the best early effort was carved out by Ryan Brunt, whose angled shot was deflected for a corner.
Plymouth 'keeper Luke McCormick tipped David Clarkson's shot around a post before the home side countered by upping the tempo in the second half.
Lewis Alessandra's incisive run teed up Rommy Boco, whose 18-yard shot hit a defender and looped over the crossbar and, from the resulting corner, Rovers somehow kept out Neal Trotman's powerful goalbound header.
Mildenhall did well to deny Argyle target man Marvin Morgan at the far post following skipper Hourihane's free-kick from the left.
But there was nothing he could do to keep out Reid as he profited from Hourihane's pacey set-piece, ensuring Rovers tasted derby defeat for the second time in four days following their midweek reverse at Bristol City.