It operates as an institutional investor on the platform, which matches lenders with borrowers.
In its annual report, TSB said the relationship aligned with its diversification strategy and was a way of supporting customers indirectly.
It said the lending facility was “relatively new”.
The Harmoney loans are unsecured.
Heartland Bank, which has a 12.6% shareholding in Harmoney, also offers money to peer-to-peer borrowers. It had lent $62m by the end of last year.