The law provides $175,000 in state aid to expand the ShotSpotter program, which currently covers three square miles. Police want to use the state money, as well as county and city funds, to expand it to 10 square miles.
Walker said the expansion will reassure families living in those areas who worry about their safety and their children's safety.
"This law won't cure everything but it's certainly a giant step forward," said Walker, who said he's seen the technology firsthand.
SpotSpotter instantly transmits the location of gunshots to officers, allowing them to respond to crime scenes quickly and to analyze shooting patterns.
The Republican governor had proposed a budget last year that eliminated $445,400 in grants that police hoped to use to expand the program. Walker's administration considered the grants an earmark for a single city, according to a Legislative Fiscal Bureau report.
Republicans who control the Legislature went along with the governor's wishes, wiping out the money in the budget that took effect in July.
Walker said he had no regrets about removing the money from the budget, but said he eventually changed his mind after he learned more about how the system works.
Police Chief Edward Flynn and Milwaukee Mayor Tom Barrett were on hand as Walker signed the measure at a Milwaukee police department, flanked by police officers and state lawmakers.
Flynn said SpotSpotter allows officers to respond to gunfire more quickly and with a better sense of where it originated. That helps officers arrive in time to gather evidence and secure the scene, and a rapid police response also reassures people in the neighborhood, he added.
He said he expected the expansion to be completed in a matter of months.