The Phase 3 clinical study of 156 patients in Japan showed that those treated with Avigan improved after 11.9 days, versus 14.7 days for a placebo group. Results of the study, conducted by subsidiary Fujifilm Toyama Chemical, were statistically significant, the company said in a release.
The announcement sent Fujifilm shares up more than 4% in Tokyo to their highest level in four months.
Former Japanese Prime Minister Shinzo Abe had touted Avigan's potential as Japan's contribution to a global race for coronavirus treatments, aiming for domestic approval in May.
The government called on Fujifilm to triple national stockpiles of Avigan, approved in 2014 as an emergency flu treatment, and pledged to donate it to countries on request.
But a lack of patients in Japan hampered clinical testing. In July, researchers at Fujita Health University said their Avigan study failed to reach statistically significant results.
In its statement, Fujifilm said it would perform further analysis of the trial data and seek to file for approval of Avigan for use in COVID-19 as early as October.
This week, the Nikkei newspaper cited ministry sources as saying approval could be granted within a month of receiving such a request.
In July, Fujifilm sold its global rights to Avigan to India's Dr Reddy's Laboratories. Already available generically in many countries under the name favipiravir, the drug has been approved to treat COVID-19 in India and Russia.
Avigan acts on the RNA of viruses, inhibiting their ability to replicate. It is the subject of dozens of trials worldwide, but concerns remain over birth defects it caused in animal studies.
Dr Reddy's shares were 0.7% higher at 0430 GMT.
(Reporting by Rocky Swift and Chris Gallagher in Tokyo; Editing by Stephen Coates)