The donation increases Kashkari's total personal contribution to the effort to $1 million. He announced the first $500,000 on Monday.
Kashkari, a former U.S. Treasury Department official, lags behind Donnelly, a Twin Peaks assemblyman and tea party favorite, in early polls. But Donnelly lacks resources for traditional advertising.
Kashkari has raised more money than Donnelly, but less than he had once expected. His second $500,000 contribution will increase the total amount he has reported raising to about $2.9 million. The former Goldman Sachs executive has put his net worth at less than $5 million.
Donnelly and Kashkari are the two main Republicans bidding to unseat Gov. Jerry Brown this year. Brown, a third-term Democrat, is widely expected to finish first in the June primary election, leaving Kashkari and Donnelly to compete for second place - and a spot in a runoff election against Brown in November.
PHOTO: Republican Neel Kashkari speaks with reporters at Oroville Dam on March 28, 2014. The Sacramento Bee/David Siders