In 2000, he moved to Portuguese Primeira Liga side Porto, where he made a strong first impression, scored the equalizing goal against rivals Sporting CP in the first leg of the 2000 Supertaça Cândido de Oliveira. He was also impressive overall in first season, as Porto captured the Taça de Portugal after winning the final 2–0 against Marítimo, Alenichev scoring the second goal. The following season, Alenichev suffered some animosity from new Porto head coach Octávio Machado (similar to his compatriot Sergei Ovchinnikov) and spent most of the first half of the season sidelined, under the shadow of Deco. When Octávio was sacked and replaced with José Mourinho, Alenichev's prospects changed. Although he was not a regular in the starting 11, he was usually the first player substituted onto the pitch, particularly when Mourinho shifted from a 4–3–3 to a 4–4–2 formation. A starting player in the 2003 UEFA Cup Final and mid-match substitute in the 2004 UEFA Champions League Final, Alenichev scored in both contests. In the former, against Celtic, he scored the second goal, following a pass from Deco; and in the latter, against Monaco, he closed the scoreline with a powerful volley shot following a deflected through cross from Derlei. This made him one of only three players to score goals in two consecutive cup finals of different European competitions, the others being Ronald Koeman and Ronaldo.