The financial gift came from Elena and Alex Gerko, founder of trading company XTX Markets.
The orchestra, whose music director is Sir Simon Rattle, said the funding will enable it to “rebuild”.
Sir Simon has previously warned the orchestra would not survive without support.
The donation was made by Elena and Alex Gerko, founder of trading company XTX Markets.
Mr Gerko is a mathematician from Russia with a wealth estimated at £464 million, according to The Sunday Times Rich List.
The orchestra previously received £846,000 from the Arts Recovery Fund.
Its managing director Kathryn McDowell said: “The importance of music and the arts has never been more apparent than in recent months, as we’ve been inspired, comforted and entertained throughout this unprecedented period.
“For the LSO, as for so many, 2020 has been exceptionally challenging, with the practical and financial impacts of lockdown, as well as ongoing social distancing, causing serious disruption to every part of our activities.”
She said the orchestra, which aims to raise £5 million by next summer, is “hugely grateful” for the donation.
The Gerkos said: “We have enjoyed hundreds of LSO concerts over the last 15 years and are very happy to help the orchestra in these difficult times.”