ExxonMobil said the work over the next two years would support around 850 local construction jobs and benefit 40 local suppliers.
Scotland's environmental watchdog the Scottish Environment Protection Agency (Sepa) launched a formal investigation into unplanned flaring at the Mossmorran plant in April following hundreds of complaints from local residents about a chemical smell and rumbling noise.
It also plans to upgrade key infrastructure and introduce new technologies that will "significantly improve operational reliability and performance".
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Jacob McAlister, plant manager at the Fife ethylene plant, said: "These planned investments demonstrate our commitment to long-term reliable operations at the site.
"While already one of the most modern plants of its kind in Europe, we are always looking for ways to improve reliability and efficiency through continued maintenance and investment in new technologies.
"Fife has a long-term future as a competitive asset, contributing to both the local and national economies."
He added: "We are committed to the highest operational and regulatory standards.
"This investment further contributes to the local economy and across Scotland through job creation and procurement contracts."
The plant has been shut since the end of August for maintenance work on two of its three boilers and is expected to reopen towards the end of the year.
It began operations in 1985 and has a production capacity of more than 800,000 tonnes of ethylene a year.
In August Sepa varied the operating permits for ExxonMobil Chemical Limited and Shell UK Limited, which shares the site, requiring them both to address the impacts of flaring and install noise reducing flare tips.