The man leading Google's most important business is leaving the company.

Google advertising chief Sridhar Ramaswamy is joining Greylock Partners, the venture capital firm announced in a blog post. He will be replaced by Prabhakar Raghavan, formerly a vice president of engineering for Google's Cloud apps, with a strong background in AI research.

Google CEO Sundar Pichai said in a statement that he "can think of no better person to lead our monetization efforts" than Raghavan.

"Prabhakar has incredible management experience, and the deep technical expertise to match," Pichai says.

Ramaswamy, whose title at Google was senior vice president of advertising and commerce, joined the company 15 years ago.

His role at the helm of the advertising unit was one of the most important positions at the company, though he himself liked to keep a low profile. Last quarter, Google parent company Alphabet made 86 percent of its $32.65 billion in revenue from advertising.

"I have been extremely privileged to have spent the past 15 years at Google, where I focused on starting and scaling new businesses and platforms," Ramaswamy says in Greylock's post. "I have always been drawn to earlier stage entrepreneurial projects and becoming a venture partner at Greylock is an exceptional way to explore these types of opportunities."

He has previously dabbled in angel investing, providing early funding to the startup Leap AI.