The finance group, which offers UK personal and commercial mortgages, bridging loans and development finance, increased lending by eight per cent month on month.
The figure for May, which includes commercial and personal lending, compares to the £277m that Together lent in April.
The data also showed that its intermediary sales team scored its highest-ever monthly lending total of £164.4m, which included company-record sales of its unregulated bridging and buy-to-let (BTL) products.
Marc Goldberg, commercial chief executive of Together, said the record resulted from the “fantastically strong” performance of sales channels across the group during the period.
He said the volumes of deals had dramatically increased, with record levels of unregulated bridging and auction finance deals, and the lender’s new digital and retention channels growing, while its back book remained resilient.
He said: “These are excellent results and are testament to the hard work of all our Together colleagues. We’re delighted to have achieved this milestone figure of £300m-a-month with other records set in terms of commercial lending.
“It is the second month in a row that we’ve lent £100m in bridging finance, which stood at £106m in May, which in itself is a fantastic achievement.”
Goldberg added: “We are working hard to build out our established values, but also thinking about our impact on the community, in terms of how we can outreach through our social housing initiative, our charitable foundation and our ESG [environmental, social and governance] strategy.
“We want to create more opportunities in the community by working with schools and local groups, but also with the young people within our own business, creating a springboard for young people’s futures.”