The 41-year-old superstar's contract with Postecoglou's Riyadh club Al-Nassr expires next June, and he has told Vogue magazine that another is unlikely.
"This is probably my last year of football, and I want to leave a spectacular legacy," Ronaldo said.
The Portugal forward, who has scored 976 career goals, married long-time partner, Georgina Rodriguez, earlier this month.
"I have my future all mapped out," he said.
"I have so many things to keep me busy that to tell you just one thing is hard. Because football could leave a big hole, you have to fill your time in various ways, not just one.
"And also have more fun, travel more, watch and play padel, which I really like, and continue to enjoy what I've earned — what we've earned. Because after all, it's been 25 years with a lot of sacrifice."
The 2026-27 SPL season is Ronaldo's 25th as a professional. During his career, he has won five Ballon d'Or awards and five UEFA Champions League titles.
Ronaldo's 28 league goals last season helped Al-Nassr become champion of Saudi Arabia in May to secure his first trophy since arriving in Riyadh, and a first league title for the club since 2019.
After returning from his wedding, he missed Al-Nassr's first game of their title defence on Saturday, watching from the sidelines as they opened with a 3-0 win over Al-Fateh.
"Cristiano is here today, and he is always present with us," Postecoglou said after the game.
Ronaldo could play against Al-Diriyah in the King's Cup on Tuesday.
"The most important thing now is recovery, especially as we have only two days after this match," former Postecoglou said.
Ronaldo's comments came days after long-time rival Lionel Messi revealed that, following his father's death, he may soon retire.
"I don't know how to carry on," Messi, 39, said. "I used to just play football, and now I'm really not sure if I'll carry on doing it for much longer."
