The synthesis of mannose-substituted tetraphenylethenes (TPEs) and their aggregation-induced emission (AIE) behavior, induced by interactions with concanavalin A (Con A), are reported. A mixture of the mannose-TPE conjugates and Con A in a buffer solution displays an intense blue emission on agglutination within a few seconds, which serves as a "turn-on" fluorescent sensor for lectins. The sensing is also selective: the conjugates act as a sensor for Con A, but do not sense a galactose-binding lectin, PNA. Con A-recognition is not affected even in the presence of other proteins in a mixture. The conjugates also exhibit high sensitivity to detect Con A. An increased sensitivity of the conjugates results if mannopyranoside substituents are linked to the TPE-core unit with a flexible chain and/or when the number of mannose residues increases.