Walker addressed the letter Thursday to Secretary of State John Kerry urging the project's approval. The letter is similar to one he sent in April and comes after the State Department found the pipeline would have little impact on the environment.
Walker says the pipeline would create up to 9,000 jobs in Wisconsin over 20 years.
The Obama Administration has delayed a decision on the pipeline for years. Environmentalists and some landowners oppose the project, while oil companies and the Canadian government support it.