HMD Global, the Finnish company that owns the rights to use Nokia's brand on mobile phones and tablets, said Chief Executive Arto Nummela is leaving the company with immediate effect.
Florian Seiche, HMD Global's president, has been appointed to the additional role of acting CEO, the company said.
An HMD spokesman said Nummela's departure was a personal matter between him and the company's board of directors, but declined to elaborate.
In a statement announcing the management changes, Sam Chin, HMD Global's chairman of the Board, said: "On behalf of the whole Board, I thank Arto for his contribution and wish him well in his future endeavours."
The company is owned by Smart Connect LP, a private equity fund managed by Jean-Francois Baril, a long-serving former senior vice president of Nokia.
HMD products are built by a unit of Foxconn after it acquired the manufacturing and distribution assets of the former Nokia phone business from Microsoft last year.
Earlier this year, HMD made a splash by reviving its classic 3310 feature handset in new brightly coloured versions. It also offers a variety of Nokia feature phones aimed at emerging markets and has begun introducing new Android smartphones.