Under FIFA rules, once Segecic is fielded in a match by Croatia, he is unable to switch back to Australia.
If Segecic didn't play for Croatia and wanted to backflip on his decision, he would need to submit a request for change of association to FIFA's Football Tribunal.
The 21-year-old played for Australia at under-17, under-20 and under-23s level and received his first senior call-up under Tony Popovic for September's window against New Zealand.
But the former Sydney FC youngster didn't play a minute across those two games and hasn't been called up since.
Segecic, who has scored six goals and notched two assists across all competitions since joining Pompey, had previously said on multiple occasions he wanted to play for the Socceroos at the World Cup.
That has clearly changed.
Segecic follows in the footsteps of Canberra-born and Australian-raised Josip Simunic, who opted to play for Croatia over Australia in 2001.
It is unclear how soon Segecic could line-up for his country of choice, given the talent available to world No.11 Croatia, who have friendlies against Colombia and Brazil this month.
The attacker follows midfielder Nectar Triantis (Greece) in attending a Socceroos camp then switching nationalities. Triantis has since debuted for Greece.
Segecic's decision is a blow to Australia's attacking stocks ahead of their World Cup campaign and before warm-up friendlies against Cameroon and Curacao in his home town of Sydney, then Melbourne, this month.
Popovic has on multiple occasions made it clear he won't be "selling" the Socceroos shirt to players eligible to play for multiple countries.
While Alex Robertson has committed to Australia, the Socceroos are still awaiting a decision from Italy youth international Cristian Volpato.
The 22-year-old knocked back Graham Arnold's approaches ahead of the 2022 World Cup.
But Volpato has warmed to the idea of playing for Australia and met with Popovic and assistant Paul Okon in Italy last month.
"We're not selling the shirt," Popovic said.
"... There were no ultimatums or any pressure - it was a discussion, and if he wants to play for Australia, whether that's, let's say, three months, six months, a year … they're young men, you have to respect the decisions they have to make.
"He's indicated he's open to the idea, now the rest is just for him to decide."
