Oregon’s Port of Portland will see its last container ship service this Saturday, when Westwood Shipping Lines makes its final call.

The small Washington-based ocean carrier has been running a monthly service at the port’s single container terminal, hauling about 150 containers of cardboard and agricultural goods like hay, grass seed and dried fruit to destinations in Japan and Korea. On Wednesday, the port announced that Saturday would be Westwood’s last day at the docks.

The decision comes just over a year after global ocean carriers Germany’s Hapag-Lloyd AG and South Korea’s Hanjin Shipping Co. Ltd. canceled their weekly services. Those services and Westwood’s together each month handled about 8,000 to 9,000 20-foot equivalent units, or TEUs, a standard measure for container cargo.

The news will further squeeze local exporters that had long used the port to get goods to Asia customers. Many of the agriculture companies in Oregon already have been sending goods out through the Washington ports of Seattle and Tacoma since the other shipping lines have left Portland, but some said they have been holding out hopes for a return to direct service.

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“That was our only gleaming little light of container cargo,” said Brenda Barnes, an export account manager with Portland-based freight services company Geo. S. Bush & Co. Inc. Ms. Barnes said she thought if Portland had at least one regular container service, other carriers might come back to the city.

Since the two large ocean carriers left Portland last year, Shelly Boshart Davis, a hay grower in the Willamette Valley, has been sending most of her crop on trucks to Seattle and Tacoma. But a few shipments still went to Westwood’s ship—a service Ms. Davis says she liked to use because it was locally-owned.

“It’s not like this is going to be a huge hit to us because we’re using all these other modes now,” said Ms. Davis, who ships about 40 to 60 containers a week to customers overseas. “But it’s one more thing to do, we have to add one more piece to the puzzle.”

Small and midsize ports like Portland are facing challenges as global ocean carriers have formed alliances and increasingly send goods on supersize ships that only the largest ports can accommodate. Portland’s harbor is only deep enough to handle ships that carry about 6,000-6,500 TEUs—one third the load of the largest vessels calling U.S. ports. Just two years ago, the Port of Portland moved about 130,000 TEUs, full of goods. So far this year, it has moved less than 1,000 TEUs.

Westwood “held on with us through tough times,” said Keith Leavitt, the port’s chief commercial officer. “While the volume wasn’t as much as the volumes we’d had before, at least it kept the terminal operational for one day a month,” he said.

A representative for Westwood didn’t respond to a request for comment. According to port officials, the monthly service was no longer financially viable for the carrier. Ocean freight rates have been plummeting this year, making the cost of the 100-mile trip up the Columbia River to Portland’s docks tougher to bear.

Port officials added they still hope to recruit another container cargo carrier to call at Portland. Mr. Leavitt said Portland has identified a handful of potential operators but many have expressed wariness of ongoing tension between the port’s dockworkers and the container terminal operator, which has caused delays in handling goods for much of the past two years.

“Losing the final service is certainly a rally cry for us and for our partners to resolve the issues, get back on the marketing trail and bring a new trans-Pacific carrier to our market,” Mr. Leavitt said.

It wasn’t clear whether Westwood may restore port calls during the heavier harvest months, later this summer. Last year, Westwood temporarily canceled service in April and returned to offer monthly service again in July.

Corrections & Amplifications:

The Port of Portland announced the departure of Westwood Shipping Lines in a statement on Wednesday, An earlier version of this article incorrectly stated it was announced Tuesday. (May 19)

Write to Erica E. Phillips at erica.phillips@wsj.com