Iwaniuk quit the position last month after almost three-and-a-half years in the role.
Nike corporate communications manager Simon Charlesworth confirmed Iwaniuk had left and that a recruitment process was underway to find a direct replacement.
Although Charlesworth would make no further comment on Iwaniuk's role or the timescale of the recruitment process, he said she had left to fulfil a personal ambition of studying for an MA abroad, a move he said she had been planning for 'a while'.
Iwaniuk could not be contacted as PRWeek went to press.
She was responsible for the full gamut of communications including marketing communications, issues and crisis management and corporate social responsibility.
Iwaniuk joined Nike from oil giant Shell International, where she worked for three years, firstly as a regional adviser for the Middle East and Africa and latterly on public affairs issues.
Before Shell she worked as an associate director in the public affairs division at Burson-Marsteller and she also represented the Confederation of European Paper Industries as a lobbyist, based in Brussels.
During her time at Nike, Iwaniuk had to deal with communications challenges surrounding controversial issues such as the firm's labour record in developing countries such as Cambodia.
In March this year, Nike created a new role to better co-ordinate its PR, advertising and marketing (PRWeek, 15 March). Duncan Mills, formerly associate media director, was promoted to EMEA marketing planning director.
It is estimated that Nike - which has its global headquarters in Beaverton, Oregon - has a 40 per cent share of the world's athletic shoe market.