Sales increased in all six Southern California counties, according to San Diego-based DataQuick.
"The market is being slowly nursed back to health by low interest rates, a modestly improved economy and, we suspect, a widening sense that the housing sector is at or near bottom," DataQuick President John Walsh said in a statement.
Last month home sales increased 20.6 percent to 22,192 properties from 18,394 a year ago. DataQuick's count includes new and previously owned houses and condominiums. Los Angeles County did even better, with sales jumping 25.3 percent to 7,496 properties from 5,983 a year ago.
Sales have now increased on a year-over-year basis for five consecutive months with last month's gain the biggest, DataQuick said.
May's regional median price increased 5.4 percent to $295,000 from $280,000 a year ago. It was up 1.7 percent from $290,000 in April, DataQuick said.
Last month's median was the highest since $295,500 in September 2010. The year-over-year gain in the May followed a 3.6 percent annual increase in April. Before then, the median had fallen year-over-year for 13 straight months.
DataQuick attributed the price increase to higher demand, a drop in the number of distressed property sales and more sales in the higher-cost coastal markets.
Last month sales in San Diego, Orange, Los Angeles and Ventura counties represented about 70 percent of all sales, up from 67.6 percent a year ago.
The higher end is finally getting some action, too. Last month sales between $300,000 and $800,000 - a range that would include many move-up buyers - jumped 23.1 percent year-over-year. And sales over $800,000 rose 11.8 percent from May 2011.
The report showed that in May:
The median price in Los Angeles slipped 1.6 percent to $315,000 from $320,000 a year earlier
Sales in Ventura County soared 43.3 percent to 993 from 693 a year ago. The median price fell slightly to $360,000 from $360,500.
In San Bernardino County, sales increased 16.3 percent to 2,702 from 2,323 a year ago. The median price rose 5.7 percent to $158,500 from $150,000 a year earlier.
Distressed sales - the combination of foreclosure resales and short sales - made up 44.8 percent of last month's resale market. That was the lowest level since the figure was 44.4 percent in March 2008.
Investor and cash-only home purchases remain near record levels.
Absentee buyers - mostly investors and some second-home purchasers - bought 27 percent of the homes sold in May. That's down from 28.4 percent in April but up from 25.1 percent a year earlier.
Buyers paying with cash accounted for 31.3 percent of May home sales, down from 32.2 percent the month before and up from 29.2 percent a year earlier.
Indicators of market distress continue to move in different directions. Foreclosure activity remains high by historical standards but is much lower than peak levels reached in recent years. Financing with multiple mortgages is very low, and down payment sizes are stable, DataQuick said.
It all adds up to a market still on the mend.
"There's still plenty of uncertainty swirling around out there," Walsh said.