Sprint has begun offering WiMAX service to its wholesale customers.

The operator will resell Clearwire’s service to outside companies in addition to its own 3G service, which it already offers on a wholesale basis.

“We have a lot of wholesale customers that are targeting niche regional markets,” says Sprint spokesman John Votava. “With 4G we’ll allow them to do that even further.”

Sprint is selling two dual-mode devices for the wholesale service, a Sierra Wireless data card and the HTC Detail, which shares an identical set of features with the HTC Evo Shift.

The operator says it will roll out additional dual-mode 3G/WiMAX devices for its wholesale customers in the coming months, including new cell phones, data cards, personal Wi-Fi hotspots and a “fixed access alternative” for DSL replacement.

Wholesale customers can choose to use their own devices running on Sprint’s network, Votava says, but the devices must pass the operator’s potentially costly and time-consuming testing process.

Votava said Sprint would announce additional customers for the wholesale WiMAX service “in the coming weeks” but declined to name specific companies.

The deal could help Sprint and Clearwire land additional WiMAX customers amid rising competition from other mobile broadband providers such as Verizon Wireless, whose LTE network is offered in more markets than Clearwire’s after less than a year in operation. Clearwire currently operates WiMAX in 71 markets, less than Verizon’s 102 LTE markets.

Votava declined to say whether Sprint would provide Clearwire with additional funding for its WiMAX network build if its latest wholesale service proved successful.

“That would have to be determined at a later date,” he said. “Clearwire is a great partner and we’re still moving forward with them on offering a great 4G network.”

Sprint gleans money from wholesale customers, and Clearwire in turn profits from additional traffic on its network generated by those customers. Sprint has agreed to pay Clearwire for its mobile broadband services on a per-gigabyte basis under a contract signed between the two companies earlier this year.

Clearwire’s investors seemed to like the news. Clearwire’s stock spiked 9 percent in afternoon trading on the New York Stock Exchange, sending the company’s shares up 19 cents at their highest. Sprint’s stock took the opposite tact, slipping more than 6 percent.