Harley-Davidson Inc. on Monday said it would add smaller, more versatile motorcycles to its lineup as it tries to attract the younger riders it needs to grow sales.

Motorcycle sales in the U.S. have been falling for three years, and Harley's U.S. retail sales dropped 6.4% in the second quarter from a year earlier.

"This is one of those key moments in our history where bold action is needed to assure our future," Chief Executive Matt Levatich said Monday during a conference call.

Milwaukee-based Harley said it would introduce a line of middleweight motorcycles starting in 2020, including its first attempt at a so-called adventure bike that can be used on streets as well as unpaved trails.

By 2022, the company plans to introduce an additional model of adventure bike, as well as several new middleweight street bikes.

Harley also plans to add small motorcycles for sale in Asia that will be built with an undisclosed partner. These motorcycles, which will feature small engines, will fortify Harley's lineup of motorcycles for customers in fast-growing foreign markets, particularly India, where sales of small motorcycles are growing at rates of 25% a year.

The company said an electric motorcycle, known as the LiveWire, is on course to be released next year. Harley also said it would redouble its efforts to sustain sales of its bigger motorcycles. Harley-Davidson's share of the U.S. market for big motorcycles slipped to 48.4% in the second quarter from 50.4% during the first quarter.

Early designs of new models including the electric bike are a significant departure from the traditional Harley styling that remains popular with a core clientele of mostly older riders. Harley hopes the new bikes will appeal to new, younger customers. Harley also plans to open smaller dealerships in urban centers.

"We know what the customer is looking for," Mr. Levatich said. "We've got lots of enthusiasm from people to bring these products to the industry."

Harley is aiming to add up to 100 new and updated models to help attract two million U.S. riders over the next decade, while also boosting international sales to 50% of Harley's total, up from 38% now.

Harley in June said it would shift production of motorcycles sold in Europe to overseas factories to avoid European Union tariffs. The EU tariffs came in response to U.S. duties on imported steel and aluminum.

President Donald Trump criticized Harley's decision to shift production. Mr. Levatich said earlier this month the move was necessary to keep costs down for customers in Europe.

Harley expects the new models and its new sales strategy to generate up to $1.5 billion in additional revenue and up to $250 million in additional pretax operating income by 2022.

Harley said it would pay to produce and market the new models with "comprehensive cost reduction and reallocation of previously planned investment and resources."

Harley plans to close an assembly plant in Kansas City, Mo., by 2019, laying off about 800 workers and moving production to its assembly plant in York, Pa., where the company expects to eventually add 450 workers.

Write to Bob Tita at robert.tita@wsj.com