A spokeswoman said that Moy is returning to the US "to be closer to his family". He will take up a new role and "help drive growth for Isobar in the US". Isobar is searching for a replacement for Moy.
Moy joined the agency in November 2015 after moving over from the US office where he was chief commerce officer. Before he joined Isobar in 2013, he worked for SapientNitro.
At the same time as Moy's arrival at Isobar, Nick Bailey was promoted to chief creative officer for EMEA. He was previously chief executive and executive creative director of Isobar UK. Bailey left the agency last year.
Isobar had a tough 2016 after it lost the Kellogg account to DigitasLBi. In 2016, the agency announced a strategic business review and sources said there were just 25 people working at the shop. However, the agency did not confirm this.
A spokeswoman said: "Steven Moy joined Isobar UK from the US, where he was the chief commerce officer, in 2015 to help shape and strengthen Isobar's brand and digital commerce strategy in the UK.
"He has led a reshaping of the UK business to ensure that it has the right capabilities to help brands to reclaim, capitalise and realise opportunities for their businesses now and in the future."