The average annual return per property for landlords across England and Wales was £21,988 in January, estate agents Your Move and Reeds Rains said.
The figure includes income from rent as well as the rising value of the property and also takes into account any void periods, when the property may stand empty. It is the highest in cash terms since August 2014.
But it does not include money which will be paid out by landlords for costs such as maintenance and mortgages or rent which is owed but goes unpaid.
The £21,988 figure is made up of £13,594 from the rising value of the property, plus £8,394 in rental income over the year to January.
There have been recent reports of landlords rushing to snap up properties ahead of a three percentage point stamp duty increase above current rates for this sector which comes into force in April.
Landlords will also see their tax breaks restricted. Wealthier landlords receive tax relief at 40% and 45% - but this tax relief will be restricted to 20% by April 2020.
From April 2016, a ''wear and tear allowance'', which allows landlords to reduce the tax they pay, regardless of whether they replace furnishings in their property, will also be replaced by a new system that only allows them to get tax relief when they replace furnishings.
Adrian Gill, director of estate agents Reeds Rains and Your Move, said: "Stamp duty premiums on new buy-to-let purchases are the rhino in the room - everyone is talking about the 1 April deadline and the extra purchase costs are perceived by some commentators as potentially hazardous.
"But this is a little simplistic. Landlords are long-term investors and generally take good advice before making a new purchase...
"Right now in 2016 the big shift is likely to be in favour of existing landlords, potentially at the expense of those planning to start up as a landlord for the first time or expand their portfolio.
"As such, it will be interesting to see how the rental market responds if there is a disruption to investment in supply."
The index is based on rents achieved on around 20,000 properties.