The world's biggest reinsurer posted a net loss of $665 million compared to a profit of $158 million a year earlier.
Swiss Re said its results were hit by exceptionally high claims for natural disaster damage including the giant earthquake and tsunami in Japan.
"In the first quarter of 2011, we experienced exceptionally high losses from natural catastrophes," chief executive Stefan Lippe said in a statement.
Swiss Re highlighted the events in Japan in March, floods in Australia and the earthquake in New Zealand.
Pre-tax losses due to natural disasters reached $2.3 billion during the first three months of the year, driving Swiss Re's property and casualty department to an operating loss of of $1.2 billion.