Third Rock Ventures, a Boston venture capital firm targeting innovative life science start-ups, said today that it has raised more than $500 million for a new investment fund.

The $516 million fund is Third Rock’s third and biggest to date. Third Rock said it closed the fund after just five months, bringing the total it has raised over the past six years to $1.3 billion.

Third Rock has invested in 31 health care companies, 22 in Greater Boston, in biopharmaceuticals, medical devices, or diagnostic technologies.

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The firm, which prefers to invest in start-ups instead of established companies, said it plans to add 16 companies to its portfolio. The company said the majority of the companies will be in Boston and have an emphasis on innovative products.

“We are grateful for the strong support for our model and significant interest from our existing and new investors,” said Kevin Starr, Third Rock partner, in a statement.“Since 2007, it has been our mission to be the preferred partner for entrepreneurs, investors, employees and industry to build great companies that discover and develop breakthrough products that make a difference for patients.”

The firm starts many of the companies it funds by scouring scientific journals and interviewing leading researchers to find promising ideas that aren’t on bigger life sciences companies’ radar. Then they develop teams of executives to build the business.

Last year, the company said it had the goal of starting five to seven new companies each year.

Third Rock, one of the largest venture capitalist firms targeting the health care sector, put up $47 million last month to launch Cambridge cancer immunotherapy start-up Jounce Therapeutics Inc. Jounce intends to develop drugs that can harness the immune system to seek out and attack cancerous cells, sometimes in combination with other medicines.

Third Rock also launched Foundation Medicine of Cambridge, which enables doctors to send in a biopsy of a patient’s tumor and have its genetic make-up analyzed. Foundation has raised nearly $100 million to date.

Third Rock and its portfolio companies have 654 total employees, 528 of which are located Greater Boston.