The annual rate of house price growth was 4.7% in May, down from 4.9% in April and 5.7% in March.
In May alone, house prices rose by 0.2%, the same as the previous month.
The Nationwide said one factor was that many people had rushed to buy houses in March because of stamp duty changes that took effect on 1 April.
The volatility generated by the changes made it difficult to gauge the underlying strength of activity in the housing market, said Nationwide chief economist Robert Gardner.
“House purchase activity is likely to fall in the months ahead, given the number of purchasers that brought forward transactions,” he added.
“The recovery thereafter may also be fairly gradual, especially in the [buy-to-let] sector, where other policy changes, such as the reduction in tax relief for landlords from 2017, are likely to exert an ongoing drag.
“Nevertheless, healthy labour market conditions and low borrowing costs are expected to underpin a steady increase in housing market activity once stamp duty-related volatility has passed, providing the economic recovery remains on track.”
