Andrejczyk claimed second place thanks to her best throw of 64.61m on her second attempt.
She was surpassed only by China's Shiying Liu, who won the gold medal with 66.34m.
Kelsey-Lee Barber of Australia captured the bronze with a best effort of 64.56m.
The 25-year-old Polish athlete secured her first Olympic medal, after finishing fourth at the previous Games in Rio de Janeiro five years ago.
This year, Andrejczyk leads the world rankings with 71.40m, which is a national record.
Andrejczyk's silver means Poland has now won eight medals in athletics competitions and a total of 12 from all the sports at the Tokyo Olympics.
She is the second Polish female javelin thrower to win an Olympic medal, after Maria Kwaśniewska, who claimed bronze at the Berlin Games in 1936.
Poland's Maria Andrejczyk celebrates winning silver in the women's javelin throw at the Tokyo Olympics on Friday. Photo: EPA/CHRISTIAN BRUNA
(pm/gs)
Source: TVP Sport, PAP