The Confederation of British Industry's (CBI) Sterling Assets report found the UK invested 449 billion US dollars (£346bn) in America at the end of 2014.
The cash injection was 76 billion US dollars (£59bn) more than the next largest investor Japan and nearly 200 billion US dollars (£154bn) more than Canada.
It means the UK holds a 15% slice of the 2.9 trillion US dollars (£2.2tn) of foreign direct investment into America, with Indian and Chinese FDI (Foreign Direct Investment) combined reaching just under 1%.
CBI international director Ben Digby said the investment was a sign that the "economic special relationship" continued to go from strength to strength.
"As the government negotiates our exit from the EU, a clear strategy will be needed to boost trade with partners, old and new, across the globe," he added.
"Markets should be carefully prioritised, in consultation with business, to lay the foundations for deep and comprehensive future trade and investment relationships, and the USA must be at the top of that list.
"The UK stood to be up to £10bn a year better off upon implementation of the Transatlantic Trade and Investment Partnership (TTIP) between the USA and the European Union, so the Government should also explore the possibility of joining the final deal, as a third party."
A total of 1,029,700 jobs were supported by British companies across America, with nearly a quarter in the manufacturing industry.
Texas was the state with the most jobs supported by British companies, hitting more than 100,000, followed by California and New York.
Meanwhile, services exports from America to the UK rose 6% to 66.9 billion US dollars (£52bn) last year, while US imports from British services also climbed 6% to 53 billion US dollars (£41bn).
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