The 21-year-old set a Premier League record for assists by a defender in the 2019-20 season.
Alexander-Arnold beat out a host of strong contenders for the award, including Manchester United pair Marcus Rashford and Mason Greenwood; Chelsea's Tammy Abraham and Mason Mount; and Arsenal teenager Bukayo Saka.
Nominees for the award had to be 21 or younger on July 1, 2019.
Alexander-Arnold was a vital part of Liverpool's runaway Premier League title in 2019-20, making 38 appearances in the league and tallying four goals and 13 assists.
The 21-year-old defender was also part of 12 clean sheets as Jurgen Klopp's team ended their 30-year wait for the top-flight title.
Alexander-Arnold broke the all-time Premier League record for assists in a season by a defender, a mark that he had set the previous season with 12.
Tuesday's award was in addition to the Premier League's Young Player of the Season, which Alexander-Arnold won last month to complete a clean sweep of prizes for y.
Alexander-Arnold's 13 assists in 2019-20 was second in the league to Kevin De Bruyne's 20, with Manchester City's Belgian star taking hom the PFA Players’ Player of the Year award on Tuesday.
Alexander-Arnold made his Liverpool breakthrough in 2016-17 and has made 93 Premier League appearances for the Merseyside outfit. As well as the Premier League crown, he has won the Champions League, UEFA Super Cup and Club World Cup with Klopp's men.
In women's awards, Chelsea star Bethany England was named PFA Player of the Year, while Manchester City's Lauren Hemp was named the PFA Young Player of the Year.
England won the prize in addition to the Barclays Player of the Season award, which she took home in July after she scored 14 goals in 15 appearances during a breakthrough season in which Chelsea won the Women's Super League title.
Hemp had already been named the PFA Young Player of the Year once, an award she took home after an impressive season with Bristol City in 2017-18.
The 20-year-old scored five goals and added five assists in the WSL last season as City finished runners-up to Chelsea.
He is a very talented player...and probably future super star.....
Hey sub done...back plz.. thanks...