Kansas City used Twitter to make the announcement of Reid's hiring.
"Andy Reid and (Chiefs CEO) Clark Hunt make it official," the Chiefs tweeted with a picture of the pair. "Reid becomes the head coach of the Chiefs."
Reid, who was sacked on Monday after 14 seasons with the Philadelphia Eagles, will be introduced on Monday. Terms were not announced.
Hunt said in a separate statement earlier on Friday they had "mutually agreed to part ways" with Pioli, though local media reports had earlier suggested that Reid's recruitment would likely result in Pioli's departure.
The 54-year-old Reid will look to rebuild a team that tied for the worst record (2-14) in the NFL but will have the top pick in next year's player draft.
Reid led the Eagles to six division titles and five NFC championship appearances during his time in Philadelphia.
