Alonso of the New York Mets trotted away with the National League honor after smacking 53 homers to break the rookie mark set by Aaron Judge (52) of the New York Yankees two years ago. Alvarez had a franchise-rookie-record 27 homers in just 87 games for the Houston Astros en route to unanimously winning the American League award.
Alonso, an All-Star first baseman, received 29 of 30 first-place votes for 148 points and easily won over second-place finisher Mike Soroka (82) of the Atlanta Braves and third-place shortstop Fernando Tatis Jr. (26) of the San Diego Padres. The right-handed Soroka received the other first-place vote.
In addition to his homer production, Alonso batted .260 with 30 doubles and 120 RBIs. The player nicknamed "Polar Bear" also won the Home Run Derby at the midseason All-Star Game.
"I really feel blessed," Alonso, who turns 25 next month, said in an interview on the MLB Network. "I have to say thank you to the New York Mets for giving me the opportunity right out of spring training and believing in me from Day One."
Soroka, 22, went 13-4 with a 2.68 ERA in 29 starts for the Braves. The All-Star right-hander struck out 142 in 174 2/3 innings.
Tatis was limited to 84 games due to injuries but the 20-year-old was spectacular when he played. He energized the Padres with a .317 average along with 22 homers, 53 RBIs and 16 steals.
Alvarez received all 30 first-place votes and 150 points to easily outdistance the runner-up, pitcher John Means (53) of the Baltimore Orioles, and third-place infielder Brandon Lowe (27) of the Tampa Bay Rays.
Alvarez, 22, fit in well with the Astros' powerful lineup by batting .313, slugging .655 and driving in 78 runs in 87 games. He is the 11th unanimous winner in AL history.
"He wants to appreciate all the help that everyone has taught him during the season, especially his teammates and the fans who follow him," Alvarez said through a translator on the MLB Network.
The left-handed Means went 12-11 with a 3.60 ERA in 31 appearances (27 starts). The 26-year-old All-Star had a solid 1.14 WHIP and struck out 121 in 155 innings.
Lowe, 25, batted .270 but was limited to 82 games due to injuries. The All-Star smacked 17 homers and drove in 51 runs.
Alonso is the sixth New York Mets player to win the award but just the second position player. Outfielder Darryl Strawberry was the 1983 winner.
Pitchers Tom Seaver (1967), Jon Matlack (1972), Dwight Gooden (1984) and Jacob deGrom (2014) are the other Mets' winners.
The Cuban-born Alvarez is the third Houston player to win the award. Jeff Bagwell won the NL award in 1991 when the Astros were in that league and Carlos Correa won the AL honor in 2015.
Chicago White Sox outfielder Eloy Jimenez (20 points) and Toronto Blue Jays infielder Cavan Biggio (seven) round out the AL top five.
Pittsburgh Pirates outfielder Bryan Reynolds was fourth in the NL balloting, followed by fifth-place Dakota Hudson, a right-hander for the St. Louis Cardinals.
--Field Level Media