Saikawa, 65, was appointed CEO after the arrest of Carlos Ghosn, who is facing allegations of financial misconduct during his tenure as former chairman of the Japanese automobile group, Efe news reported.
Although Saikawa had announced his intention to step down from his post after the turbulence caused by Ghosn's arrest had died down, his departure was allegedly sped up by other financial irregularities involving him.
There was yet to be an official confirmation of Saikawa's resignation as chairman and CEO of Nissan Motor but on the company's website on Monday, Yasuhiro Yamauchi appeared as Acting President and CEO.
Yamauchi, 63, who is also Chief Operating Officer (COO) of Nissan Motor, will continue to hold that post.
The firm intends appoints a new CEO at the end of October when the selection process is complete.
Saikawa's resignation comes shortly after he admitted that his income had been boosted by speculating on the terms of a salary bonus which was subject to the performance of the company's shares.
Although Nissan Motor's board did not believe that these actions were illegal, it did consider them serious enough to ask Saikawa on September 9 to step down from his position a week later.
Yamauchi joined Nissan in 1981 and became the manager of the Purchasing Administration Department in 1995. Since May, he has been COO and director and representative executive officer since June.
( With inputs from IANS )