The award was in the category for new members of Opportunity Now, which was making its sixth annual awards. JP Morgan won because it had set up a War for Talent steering group, to help devise policies and activities to help meet its commitment to diversity.
Kate Grussing, a vice-president at JP Morgan said: "Winning this award is a great accomplishment for JP Morgan. It is a symbolic step for women in the City of London.
"We hope it will galvanise our competition to address women's issues that exist across our sector. We recognise that we will still have much to do and take that responsibility seriously."
The awards patron, Cherie Booth QC, praised the nominees and winners for their efforts to promote diversity and gender equality as core business issues.
She said: "Now more than ever there is a compelling business case to increase women's representation at all levels women are a large and growing percentage of the available workforce."
Gender equality and diversity are increasingly being seen as key issues that need to be addressed by all industry sectors. The investment banking sector is widely regarded as lagging behind many of them, including the retail banks.
Other winners were drawn from the public, private and education sectors, including BUPA, HSBC, Durham-based Bishop Auckland College and City of Liverpool
Opportunity Now is run by Business in the Community, whose members include 75% of the FTSE 100 companies. Its investment banking members include Close Brothers, JP Morgan, Goldman Sachs, UBS Warburg, Morgan Stanley Dean Witter, Merrill Lynch and Lehman Brothers.