The prognoses of patients with non-small-cell lung cancer (NSCLC) harboring anaplastic lymphoma kinase (ALK) gene rearrangement have dramatically improved with the use of ALK tyrosine kinase inhibitors. Although immunological and nutritional markers have been investigated to predict outcomes in patients with several cancers, their usefulness in targeted therapies is scarce, and their significance has never been reported in patients receiving first-line treatment with alectinib. Meanwhile, neutrophil-to-lymphocyte ratio and platelet-to-lymphocyte ratio (PLR) has been investigated during crizotinib treatment. This multicenter retrospective study evaluated 42 consecutive Japanese patients with ALK-positive NSCLC who received first-line treatment with alectinib. Immunological and nutritional markers were evaluated at baseline and 3 weeks after alectinib introduction, and their significance in predicting progression-free survival (PFS) was explored. PFS duration was significantly associated with baseline PLR (hazard ratio (HR): 2.49, p = 0.0473), systemic immune-inflammation index (SII; HR: 2.65, p = 0.0337), prognostic nutrition index (PNI; HR: 4.15, p = 0.00185), and the 3-week values for SII (HR: 2.85, p = 0.0473) and PNI (HR: 3.04, p = 0.0125). Immunological and nutritional markers could be useful in predicting the outcomes of first-line treatment with alectinib. Since PLR and SII consist of platelet counts, platelet count could be an important constituent of these markers.