On January 23, 1958, Pearson was sent with Norm Zauchin to the Senators in the same trade that brought infielder Pete Runnels to the Red Sox. He wrote Senators' owner Calvin Griffith a letter that month, asking permission to report to camp early, as he wanted to make a good impression despite his short size. Pearson won a spot as the Senators Opening Day centerfielder and played the position all year for Washington. Hitless in his first three games, his first major league hit came on April 19, a single against Mike Fornieles in a 4–3 victory over the Red Sox. On July 28, he had three hits and hit his first major league home run against eventual Hall of Famer Early Wynn in a 6–5 loss to the Chicago White Sox. On September 2, he hit an inside-the-park home run against Hal Brown, driving in three runs in a 4–3 victory over the Baltimore Orioles. Three days later, he hit a two-run home run against Bob Turley in a 6–3 victory over the New York Yankees. In 1958, he won both the Major League Baseball (MLB) Rookie of the Year and the "Sporting News" Rookie of the Year Awards in the American League (AL). Pearson played 146 games for the Senators that year, batting .275 with 63 runs scored, 146 hits, three home runs, and 33 RBI.