UNHCR Chief Communications and Spokesperson Melissa Fleming delivered the data, accounting for 200,000 landings in Greece and 110,000 in Italy, at a press briefing in Geneva, Switzerland.
"This represents a large increase from last year, when around 219,000 people cross the Mediterranean during the whole of 2014," Fleming is quoted on the UNHCR website as saying.
The UNHCR spokeswoman appealed to "all governments involved" to respond to the migrant crisis and "act with humanity."
The latest incidents involved the sinking of two boats near a northwest Libyan coastal town on Thursday, carrying a combined total of approximately 450 migrants.
The International Organization for Migration (IOM) has recorded 2,373 cases of migrants who died while attempting to reach Europe by sea this year. Thousands more have been picked up from dinghies and brought to temporary asylum centers in southern Europe.