The value of private equity-backed M&A is down 50 percent to $1.32 billion in 2013 year to date, according to Mergermarket data.
During the full year 2012, the figure was $2.69 billion and in 2011, $3.46 billion.
The figures exclude real estate and infrastructure deals.
Deal volume was also down. Twelve deals were completed this year compared to 19 for the full year 2012.
Malaysia had the steepest drop in investment, plunging to $211 million across three deals year to date, compared to $1.73 billion across four deals in 2012.
Indonesia private equity deals fell to $30 million from $222 million last year.
The Philippines and Vietnam had an increase in deal value to $373 million and $250 million, respectively. Both countries had no private equity investment in 2012.
A huge gap exists between private equity-backed and overall M&A.
Total M&A in Southeast Asia (to October 1) reached $49.8 billion across 273 deals, the largest amount of value and volume ever, according to a separate report from Mergermarket.
Thailand had a 200 percent increase in total M&A to $ 16.1 billion during the first three quarters of the year, but had zero private equity-backed M&A (to the end of November).