SAN FRANCISCO—Democratic Sen. Bernie Sanders is seeing a surge of financial support from companies in Silicon Valley and is now out-raising presidential rival Hillary Clinton at the largest tech firms there, a boost that likely helped the Vermont senator’s fundraising outpace Mrs. Clinton’s last month.

In the last three months of 2015, Mr. Sanders drew nearly $105,000 from employees at the five largest technology companies in Silicon Valley. That was about four times the amount he raised from those firms in the first quarter...