TOKYO—Struggling with declining profit and a surging Amazon.com Inc., internet retailer Rakuten Inc. is leaning on its credit cards—the ones it issues to millions of Japanese.

Rakuten, which operates an internet mall, said Thursday its credit-card business posted 45% growth in operating profit for the July-September quarter, helping counteract a decline for the third quarter in a row in the company’s domestic e-commerce business.

The company, founded by Hiroshi Mikitani in 1997, now gets nearly 40% of its revenue from financial services, operating Japan’s biggest internet bank and the third-largest credit company by transaction value. That helps it counter headwinds from slower growth and intensifying competition from Amazon in attracting virtual shoppers.

Rakuten’s business model revolves around a point-based membership program that encourages people to use services such as travel booking and food delivery and pay for them with the company’s card. They can use their points on those services and purchases on the internet mall.

“Our credit-card users are like a wall, protecting us” from competitors, said Masayuki Hosaka, Rakuten’s vice chairman, who oversees the finance business. He predicted that the Rakuten card, held by over 13 million people, would reach No. 1 in Japan by transaction volume in about two years.