Lured by 21 miles of Pacific coastline, tech titans, overseas oligarchs and others are paying from $20,000 to $750,000 a month to rent summer homes in Malibu, Calif.

This summer, Kurt Rappaport is renting out his oceanfront estate in Malibu, Calif., for a price he sees as a relative bargain: $750,000 a month.

Mr. Rappaport, co-founder of Westside Estate Agency, a real-estate brokerage, set his fee well under what it would cost to stay at the best suite at the Beverly Wilshire Four Seasons for a month or to charter a yacht.

“Presidential suites don’t have screening rooms, your own gym, or a 168-foot-long pool. It’s kind of a no-brainer,” said Mr. Rappaport, who is 45 years old. The 22,000-square-foot home, completed in 2014, is already booked for the summer, he said, declining to name the tenants.

Malibu, an upscale beach community with 21 miles of Pacific coastline, was once primarily a favorite summerlong getaway for Hollywood power brokers. In recent years, the community has undergone a sea change, local homeowners and real-estate agents say. Monthly rents are soaring, with the biggest demand for summer beach homes coming from the financial and tech industries, said Tony Mark, a broker associate at Sotheby’s International Realty in Malibu. He estimates that about half of his clients come from Los Angeles, a quarter from the East Coast and another quarter from oversees.

Five years ago, “the world’s wealthiest started paying attention,” said Barry Kinyon, a broker associate at Sotheby’s International Realty in Malibu who has been offering summer rentals for 20 years. Last year, Mr. Kinyon had clients—“they were foreigners, oligarchs,” he said, declining to name them—who paid $33,000 a night for a Malibu beach compound for about a month.

They chartered a helicopter to take their kids to Legoland and hired top chefs from Beverly Hills to put on dinner parties, Mr. Kinyon said.

“It’s a new market that’s emerging,” said Johnny Steindorff, a partner at 4 Malibu Real Estate, citing Mr. Rappaport’s house and another estate listed for $350,000 a month.

More typical rental prices start around $20,000 a month in July or August for a two- to three-bedroom oceanfront condo and $35,000 a month for a “nice luxurious beachfront house,” said Mr. Steindorff.

Houses with privacy hedges and fences, tennis courts and pools, which are rare in Malibu, tick the price upward. Another selling point: proximity to Nobu Malibu, a famous sushi restaurant.

Among the most coveted amenities: private entry or “key access” to the beach. These are beaches where the public cannot access the entryway because it is locked by a gate to which only residents have the key. Santiago Arana, managing partner at the Agency, a Los Angeles brokerage, said a couple of years ago, renters paid $200,000 a month for a client’s house with private beach access.

Jon Waterman has rented in Malibu for the past three summers, commuting from his home in L.A.’s Hollywood Hills neighborhood for long weekends. While in the past he rented for a month or two, this year he is planning on staying the whole summer. His budget: $50,000 to $60,000 a month. He’s looking for a two- to three-bedroom property on Carbon Beach with an “open floor plan, great deck space and plenty of great, sandy beach,” said Mr. Waterman, chief executive officer of Ad.net, an Internet advertising firm.

Since he has been renting, Mr. Waterman said he has noticed more foreigners in the Malibu scene, and that prices have risen. “Malibu has slowly become a little bit of the L.A. Hamptons,” said Mr. Waterman, who is 38 years old.

Alison Betts, an agent for Teles Properties, also noted the price creep. “The same listing that would have rented for $65,000 a month at the last peak [around 2006] could be $100,000 today,” she said.

Renters can expect to pay the most in prime locations on Carbon Beach, Malibu Road, the Malibu Colony and Broad Beach. These have the most cachet because they are “off the highway, quieter, have easier parking, nice walking streets and are close to the nicer commercial areas,” Mr. Mark said.

However, erosion has wiped away the sand on parts of Colony and Broad beaches, making some parts less desirable, Mr. Kinyon noted.

Carbon Beach “has its own allure,” said Mr. Mark. It boasts the easiest commute to more central parts of Los Angeles, and is where David Geffen has an estate and Oracle founder Larry Ellison owns multiple properties; it’s also the closest to Nobu.

Lela Becker has been renting on or near Carbon Beach for the past 10 years, paying between $20,000 and $30,000 a month, she said. Her husband, mother, sister and other relatives make up a core group of eight who sleeps in the house, but daily gatherings of 15 to 25 family members and friends are common, she said.

There they spend time surfing, paddle boarding, building bonfires, barbecuing and “hanging on the beach,” said Ms. Becker, the 40-year-old co-founder of Mother, a company that makes high-end denim jeans.

The robust monthly market has encouraged landlords who used to rent long term to concentrate on short-term rentals.

Geraldine Gilliland, a Los Angeles restaurateur, bought a 2,600-square-foot Malibu Road house in 2012 for $4.42 million, according to public records. The house is currently offered for $35,000 a month for June or September, and $50,000 a month for July or August, said Mr. Kinyon, the listing agent. Ms. Gilliland said that she initially rented it long term, but recently began renting it seasonally.

“I kind of like the idea of family and friends using it when it’s off season,” said Ms. Gilliland, who lives on a ranch in Malibu about 10 minutes away.

HomeAway, a vacation-rental website based in Austin, Texas, lists 276 properties in Malibu, more than twice as many as it did in 2009; over half of them charge over $500 a night, according to Jon Gray, chief revenue officer.

Airbnb says there are “more than 300” Malibu properties on its site. Last year, Airbnb and the City of Malibu reached an agreement in which the company would collect and pay the city a 12% transient occupancy tax on income from listings on the site. Airbnb, which represents “the lion’s share of properties, at least that we know of,” is the only website to have signed such an agreement with the city, said Matt Myerhoff, a spokesman for the City of Malibu.

Stacy Clunies-Ross manages two properties in Malibu owned by her family, including a 5,700-square-foot house on Broad Beach that fetches about $100,000 a month in the summer. To fill in off-peak periods, she lists the house for $3,000 to $5,000 a night on short-term rental sites. She also rents the house out for film shoots: HBO is filming a project there, she said. When the house is not leased, her parents stay there, she said.

Four years ago, the family was offered $16 million for the house, but they turned it down, Ms. Clunies-Ross said. “It makes more money this way.”

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