The 21-year-old's inclusion, despite playing only three matches for Sampdoria following his move from Slovan Bratislava in January, was the only real surprise as coach Jan Kozak announced his final 23 for the tournament in France.
Skriniar won his first cap in the 3-1 win over Georgia on Friday and came on as a substitute in Sunday's 3-1 defeat of Germany.
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"I needed an alternative in the centre of the defence and Milan Skriniar meets all the criteria," Kozak told reporters.
"It's a little bit of a worry that he hasn't played match but on the other hand, he trains every day with top quality players. It's hard to say whether he would have been better off remaining in our league."
Defender Lukas Tesak, midfielders Erik Sabo and Matus Bero and forward Adam Zrelak were the four players cut from the provisional squad.
Kozak told reporters that defenders Norbert Gyomber and Tomas Hubocan had recovered from toe and heel injuries respectively.
Slovakia, making their debut at the European championship and beaten only twice in the last two years, face Wales, Russia and England in their first-round group.
Napoli's Marek Hamsik and Liverpool's Martin Skrtel are the top names in a solid, workmanlike team.
Goalkeepers: Matus Kozacik (Viktoria Plzen), Jan Mucha (Slovan Bratislava), Jan Novota (Rapid Vienna)
Defenders: Peter Pekarik (Hertha Berlin), Milan Skriniar (Sampdoria), Martin Skrtel (Liverpool), Norbert Gyomber (AS Roma), Jan Durica (Lokomotiv Moscow), Kornel Salata (Slovan Bratislava), Tomas Hubocan (Dinamo Moscow), Dusan Svento (Cologne).
Midfielders: Marek Hamsik (Napoli), Juraj Kucka (AC Milan), Miroslav Stoch (Bursaspor), Vladimir Weiss (Al Gharafa), Robert Mak (PAOK), Patrik Hrosovsky (Viktoria Plzen), Jan Gregus (Jablonec), Viktor Pecovsky (Zilina), Stanislav Sestak (Ferencvaros), Ondrej Duda (Legia Warsaw).
Forwards: Michal Duris (Viktoria Plzen), Adam Nemec (Willem II Tilburg)
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