Police said the man asked for a cup of hot water in the Starbucks at the downtown Horton Plaza on Friday.
The water was free, but when he started taking condiments from a nearby table, he was told they were only for paying customers.
"The lady in the back screamed, and he threw the hot cup at her, directly to her chest and face," described a witness.
The victim was treated for burns to her face and chest. The suspect, identified as 58-year-old Dennis Christopher Lee, is believed to be a homeless veteran. He was being held on $8,000 bail.