One of the most representative morpho-phonological features of Finnish is the existence of vowel harmony. Back vowels (a, o, and u) and front vowels (ä, ö, and y) cannot appear in the same monomorphemic word (e.g., PÖYTÄ [table] but not POYTÄ)—the vowels e and i are considered “neutral” and can accompany either front or back vowels (e.g., PELÄSTYÄ [get frightened]). Previous research has revealed that native speakers of Finnish use vowel harmony to help segment multilexeme compound words where each lexeme may differ in vowel harmony (e.g., työmaa = työ+maa [workplace]). In Experiments 1 and 2, we examined whether vowel harmony has an effect on the initial moments of monomorphemic word processing using the masked priming technique (lexical decision: Experiment 1; naming: Experiment 2). A target word (e.g., MÄNTY [pine]) could be preceded by a harmonious or disharmonious prime (mänty-MÄNTY vs. manty-MÄNTY; mönty-MÄNTY vs. monty-MÄNTY). As further controls, we also included a comparison with two harmonious conditions differing in the presence of a diacritical letter (mänty-MÄNTY vs. menty-MÄNTY) and a letter similarity comparison with disharmonious primes (manty-MÄNTY vs. monty-MÄNTY). To further examine whether vowel harmony has an effect at later phases of visual word processing, Experiment 3 compared the recognition of harmonious and disharmonious pseudowords in a single-presentation lexical-decision task (e.g., HÖPEÄ vs. HOPEÄ; baseword: HÄPEÄ [shame]). We found slower responses for harmonious than for disharmonious pseudowords. Taken together, these findings reveal that, while Finnish readers are sensitive to vowel harmony, this effect does not occur in the initial stages of processing.