Cincinnati received outfielder Hector Rodríguez and right-hander Jose Acuña.
Naquin, a first-round draft pick 10 years ago, was batting .246 with seven homers and 33 RBI in 56 games for the struggling Reds, who are last in the NL Central. He homered and drove in two runs Thursday in a 7-6 loss to Miami.
As the NL East leaders look to add offense, he provides another left-handed bat and depth in an outfield that already includes All-Star right fielder Starling Marte, center fielder Brandon Nimmo and left fielder Mark Canha.
All-Star Jeff McNeil also plays left field occasionally when he's not at second base.
The Mets view the 31-year-old Naquin as a good all-around player who can start a couple of times a week against right-handed pitching, against whom he has solid career numbers (.280/.332/.473 with 50 homers). They had been interested in him since last month and talks intensified when the clubs began exchanging names a few days ago.
Naquin has a $4,025,000 salary and is eligible for free agency after the season. His acquisition pushes New York's luxury tax payroll above $290 million, the new fourth tax level named after free-spending Mets owner Steve Cohen.
The 28-year-old Diehl has an 11.12 ERA in five big league games covering 5 2/3 innings this season. He earns the $700,000 minimum while in the majors.
Diehl, a Cincinnati native, has made 21 career major league appearances. He was 2-1 with a 4.24 ERA and 30 strikeouts over 23 1/3 innings at Triple-A Louisville this year.
To make room on the roster, New York designated right-hander R.J. Alvarez for assignment.
The 18-year-old Rodriguez hit .356 with three homers and 16 RBIs in 26 games in the rookie-level complex league in Florida. He also appeared in two games with Class A St. Lucie this month, going 1 for 7 with a walk. He can play all three outfield spots and third base.
The 19-year-old Acuna is 3-0 with a 2.67 ERA in eight minor league games this year, including five starts. He tossed three scoreless innings in his second start with St. Lucie on Tuesday.
Major League Baseball's trade deadline is Tuesday.