SAN FRANCISCO — Joseph Siino, the lawyer who built Yahoo’s intellectual property department, is leaving the struggling search giant.

Yahoo has lost a number of executives this year, its stock continues to slump, and it began the process of laying off 1,500 workers last week.

Siino, who wasn’t part of the cuts, said he’s leaving to join the growing patent monetization industry, which includes businesses like patent brokerages.

“Many operating companies are struggling right now, but many IP companies are booming,” Siino said. “I’ve been presented with a couple of opportunities that are really taking advantage of this unique time in the evolution in the IP industry.”

Siino wouldn’t reveal his immediate plans, saying nothing would be finalized until the new year.

Hired in 2005 to build Yahoo’s IP department, Siino assembled a group of about 35 at its height. Siino said he felt his work was done because the group, now about 30, could stand on its own and because there wasn’t a chance to continue building, given the tough times at Yahoo.

“Because the last year or so has been a challenging time for the company, it’s a little difficult to build beyond what we have now, especially in this business climate,” he said. “In the current business climate, I don’t think I would’ve been able to take the group to the next level, so there wasn’t a lot of further building to be done.”

When Siino joined the company, Yahoo’s stock was above $30 a share. Now, after falling behind Google in the search market and rebuffing a takeover bid from Microsoft this summer, the company’s stock price is stuck at around $13 a share. Yahoo CEO Jerry Yang stepped down last month.

Siino declined to comment on the turmoil or layoffs at the company.

Siino’s group focused exclusively on intellectual property at Yahoo, building the patent portfolio, cutting licensing deals, and handling IP litigaiton.

“Joe did a good job cleaning out [patent applications] that they didn’t need and filing for more strategic things and acquiring things where they were needed through acquisitions,” said Yar Chaikovsky, a patent litigator with Sonnenschein Nath & Rosenthal. “He made a lot of really good hires and built up a pretty strong IP group.”

Chaikovsky should know. He worked as an in-house IP lawyer at Yahoo during the dot-com bubble. He said getting an Internet company like Yahoo to understand the importance of intellectual property was always a challenge.

“I was evangelizing like Oral Roberts about the importance of IP,” Chaikovsky said. “When I got there, Yahoo had six issued patents and 14 applications.”

When Siino got there four years later, Yahoo was filing about 100 patent applications a year. Now Yahoo is filing 1,000 patent applications a year, Siino said.

Although that’s not as many as such massive tech companies as Microsoft or Cisco Systems, patent lawyers say it’s a big number for Internet companies, which have always been a little allergic to being viewed as tough enforcers of intellectual property.

In his farewell e-mail to his fellow Yahoos, Siino said that the “Yahoo IP and innovation machine is roaring, and Yahoo now has billions of dollars’ worth of IP assets.”

One of the biggest deals that Siino helped broker was the $1 billion joint venture with Alibaba.com, which in 2005 created China’s largest Internet company with the help of Yahoo’s IP.

“In the early stages there was a lot of education, but, in the end, Yahoo had become a much more sophisticated company in terms of IP,” Siino said.

“We knew we’d made it when I said at a meeting how something may not be related to IP, and Dan Rosensweig, who was COO then, turned to me and said ‘everything is related to IP.’”

Siino said he didn’t know who would take his place, and calls to Yahoo General Counsel Michael Callahan and others in the legal department were not returned.

Before joining Yahoo, Siino had worked at IP consulting shop Inflexion Point Strategy. He also headed Brobeck, Phleger & Harrison’s IP strategy and technology transactions group.

Before heading on to new horizons, Siino is taking some time off to travel to Rome and his ancestral home, Sicily. He’ll be visiting relatives there and, of course, eating some of “the most wonderful swordfish in the world.”