Booker, 21, tweeted a photo of himself smiling as he signed the contract Saturday night, moments before the Suns announced the deal, which makes him the highest-paid player in the franchise's history.
Booker averaged 24.9 points, 4.7 assists and 4.5 rebounds in his third season with Phoenix and shot 38.3 percent from 3-point range.
He has been general manager Ryan McDonough's most decorated draft pick in Phoenix, taken as a freshman out of Kentucky who dropped to No. 13 in the 2015 National Basketball Association draft.
The contract takes effect in the 2019-20 season.
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"I've been treated right from the day I was drafted, even when I was a rookie and didn't play much", said Booker, according to Scott Bordow of The Arizona Republic.
In his tweet, he said he was "humbled and honored" to sign the deal.
That may be a bit optimistic, but the team added to its talented core of young players big-time when it selected centre Deandre Ayton with the No. 1 overall pick in the draft.
Early in the first quarter of the Summer Suns' opening matchup against the Dallas Mavericks, guard Shaquille Harrison found Ayton in transition down the floor, lobbing it up and allowing the big man to slam it home.