Secretary of State Kevin Shelley, who announced the new rule Friday, also introduced stricter requirements for testing and auditing software used to record and tabulate votes in the nation's most populous state.
Because California commands a sizable share of the market for voting machines, the move may prompt changes in equipment being adopted nationwide as local governments rush to modernize voting systems.
Shelley wrote in the announcement that he is requiring receipts not because "voting systems are inherently insecure, they are not, but rather because people understandably feel more confident when they can verify that their votes are being recorded as intended."
Diebold Election Systems and Sequoia Voting Systems, two of the nation's largest manufacturers of voting equipment, said they would comply with the new regulations.
Executives at the companies have denied that the integrity of the vote is at risk without paper receipts, and they have been fighting such a requirement for months.
But computer scientists and voting rights advocates have been warning that at least 50,000 paperless terminals nationwide expose the vote to hackers, software bugs and mechanical breakdowns. They say other states will likely adopt regulations similar to California's new standards.
Congress has set aside $3.9 billion for states to overhaul voting systems, and hundreds of counties are considering paperless terminals. Elections experts say as many as 75 percent of voters nationwide will cast ballots electronically by 2010, up from about 10 percent in the 2000 presidential election.
"I think this is going to be a tidal wave from California eastward - it's going to get a lot of attention and may change a lot of minds," said Stanford University computer science professor David Dill. "California is the biggest market for election equipment. No company can blow it off, so they're going to have to accommodate this requirement."
The announcement comes less than three weeks after a state task force on voting began an investigation into allegations that uncertified software was used in California's Oct. 7 gubernatorial recall election.
According to Shelley's nine-page document, all counties that purchase new touch-screen terminals must provide a "voter verified paper audit trail" starting in July 2005.
Voting equipment companies must retrofit systems already being used in at least four California counties with printers by July 2006. This makes California the first state to force equipment vendors to retrofit machines already purchased or installed.
Oregon, New Hampshire and Wisconsin require paper printouts in electronic voting, but they have not deployed touch-screen machines except in small experiments.
Nearly 1.5 million Californians, one in 10 registered voters, cast ballots on touch-screens in the recall election. Four in 10 will likely use electronic systems in California's March 2004 elections.
"This is historic - a huge development and big step toward reliable and transparent voting systems," said Kim Alexander, president of California Voter Foundation.