He died in the southwestern city of Wrocław after collapsing during a running training session, state news agency PAP reported.
Magiera had served as assistant to national team coach Jan Urban since July last year and was widely regarded as one of the most respected figures in Polish soccer.
'Coach, this was not how it was supposed to be'
Poland captain Robert Lewandowski paid tribute, writing: "Coach, this was not how it was supposed to be,” and offering condolences to Magiera’s family.
European football’s governing body UEFA said it was "deeply saddened" by the news and extended condolences to Poland’s soccer community, as well as to Magiera’s family, friends and colleagues.
'Farewell, Poland thanks you'
Poland’s President Karol Nawrocki said he received the news "with deep sorrow," adding in a social media post: "Farewell, Poland thanks you."
Sports Minister Jakub Rutnicki described Magiera as "a wonderful person and an excellent coach."
Magiera was a former midfielder who played for clubs including Raków Częstochowa, Legia Warsaw and Widzew Łódź. He won two Polish league titles, as well as the Polish Cup and Super Cup, with Legia.
He also represented Poland at youth level, winning the European under-16 championship in 1993 and later captaining the under-17 team to a fourth-place finish at the Under-17 World Cup.
After retiring as a player, he moved into coaching, working with youth national teams and several top-flight clubs. He led Legia Warsaw to the Polish championship in the 2016/2017 season and guided the club in the Champions League, where it finished third in its group.
He later coached Poland’s under-20 and under-19 teams and led Śląsk Wrocław to a runners-up finish in the domestic league.
Magiera was part of Urban’s staff during Poland’s recent World Cup qualifying campaign, in which the team lost a playoff final to Sweden and failed to secure a place at this year’s tournament in North America.
In tribute, all matches in Poland’s top league and several lower divisions, as well as women’s competitions, will be preceded by a minute of silence over the weekend, the PZPN said.