The new fixed rate deal starts at 3.14 per cent and is available on a loan-to-value (LTV) of up to 70 per cent.
At the same time the lender has cut the rates on its second charge two-year fixed rate deal to 4.24 per cent, which is available at an LTV of up to 65 per cent.
On buy-to-let, the bank now offers a five-year fixed rate product starting at 3.34 per cent and available on up to 70 per cent LTV.
Second charge buy-to-lets, meanwhile, are available on up to 75 per cent LTV.
The lender said it had seen good growth in its long-term lending division over the past year, including from partnerships with TMA Club, Intrinsic Financial Services, SimplyBiz and Brilliant Solutions, which made the overhaul possible.
Matt Andrews, managing director of mortgages at Masthaven, said: "We have always applied a common-sense approach to lending at Masthaven, which means we constantly review and update our product lines to ensure our customers benefit from competitive and affordable lending options.
"We are delighted to be able to reduce our rates to offer a competitive five-year fixed rate deal and at the same time launch a very competitive three-year fixed rate mortgage for our first-charge residential borrowers."
Masthaven launched as a retail bank in 2016, having provided bridging loans and secured lending since 2004.
Matthew Fleming-Duffy, director at Cherry Mortgage & Finance, said he was pleased to see another lender enter this market but added there were probably better rates out there.
He said: "Fundamentally any shaving of rates is positive on the assumption these lenders are getting their numbers right in terms of their approach to risk.
"More lenders approaching this specialist area of lending where people have been trapped is positive too.
"In terms of the pricing it is decent enough but I doubt it is market leading."