SAN FRANCISCO—When Alexandra Goldman got notice of her rent increase—from $6,000 a month on a five-bedroom house she shared with roommates to $11,000—she says she was in disbelief.

"We knew immediately we were not going to be able to pay that much money to live there," said Ms. Goldman, a 28-year-old planning consultant whose share of the rent was about $1,000 a month. After receiving the notice in October, she said the house's occupants ended up dispersing to other rentals. She is living with other roommates now and paying about $300 more than she had before. Her former landlord didn't immediately respond to requests for comment.

Welcome to what is arguably one of the worst cities in America to be a renter, but among the best to be a landlord and apartment investor. San Francisco led the top-50 U.S. metropolitan areas in average rent growth during the second quarter, jumping 7.8% to $2,498, while Oakland was No. 2 at a 6.9% increase, and San Jose was in fifth place at 5%. The 6.8% increase for the combined San Francisco Bay area was more than double the nation's 3.1% increase, according to preliminary estimates by MPF Research, a market-research firm in Carrollton, Texas.

The rent increases have investors rushing to purchase existing properties. San Francisco-based Ridge Capital Investors, for example, has acquired nearly 500 units throughout the area since 2011, including a 45-unit apartment complex in San Mateo, a city south of San Francisco, for $10.95 million in November. Trevor Wilson, managing director of Ridge Capital, said his company has competed against as many as 30 bidders on multifamily properties in recent months.

"We've been trying to find more [properties], but there's not a lot available," said Mr. Wilson, who added his firm is spending about $1.5 million to refurbish units at the 34-year-old Mariner's West Apartments in San Mateo so rents now well below market levels can be raised. Tenants there are now paying as much as $1,000 below the market rate of $2,300 to $2,400 a month for that area, Mr. Wilson said.