The 19-year-old secured Poland's first medal of the Games in the normal hill event at Predazzo, finishing just 3.4 points behind Germany's Philipp Raimund.
Tomasiak jumped 103 metres in his first attempt and 107m in his second, earning 270.7 points overall.
Raimund took gold with jumps of 102m and 106.5m for 274.1 points.
Switzerland's Gregor Deschwanden and Japan's Ren Nikaido shared bronze, both finishing 4.7 points behind Tomasiak.
The medal is Poland's 10th individual ski jumping honour in Winter Olympic history.
Previous champions include Wojciech Fortuna in 1972 and Kamil Stoch, who won three golds in 2014 and 2018.
Adam Małysz remains Poland's most decorated ski jumper with three silvers and a bronze across the 2002 and 2010 Games.
Congratulations pour in
Polish politicians have congratulated Kacper Tomasiak following his silver medal win.
"We have a medal. Kacper Tomasiak – bravo," wrote President Karol Nawrocki.
Prime Minister Donald Tusk added: "Didn't I say so! Bravo Kacper!"
Tusk had earlier posted that he believed in Poland's ski jumpers and was hoping for good news from Predazzo, "despite all signs in heaven and on earth".
Kacper Tomasiak is now the youngest Polish athlete to win a medal at a Winter Olympics.
The 19-year-old, who made his World Cup debut in 2025, had never previously finished on an individual World Cup podium before Monday's result.
The Milan-Cortina Winter Olympics run until February 22, with Poland represented in 12 of the 16 sports on the programme.
(ał)
Source: IAR