Fundraisers have persuaded three major organisations - the Foyle Foundation, the European Regional Development Fund and the Bridge House Trust - to hand over the best part of £1 million.
This new injection of cash brings the total raised to £7.5 million, with the £10 million target getting ever closer.
Robbrecht en Daem's planned expansion (pictured) will include moving galleries into the neighbouring Passmore Edwards library building - which will itself move to a new home in Mile End.
There are also proposals to build a street-level restaurant, expand the bookshop and provide public access to the gallery's archive for the first time in its 100-year history.
Whitechapel Gallery director Iwona Blazwick was clearly delighted with the fresh investment.
'These new funds bring the current total to over £7.5 million of the Whitechapel's £10 million target for the realisation of our plans to incorporate the Passmore Edwards Library next to the Gallery into the new scheme,' she said.