Unlike signatures in a single-party setting, threshold signatures require coop- eration among a threshold number of signers each holding a share of a common private key. Consequently, generating signatures in a threshold setting imposes overhead due to network rounds among signers, proving costly when secret shares are stored on network-limited devices or when coordination occurs over unreliable networks. In this work, we present FROST, a Flexible Round-Optimized Schnorr Threshold signature scheme that reduces network overhead during signing opera- tions while employing a novel technique to protect against forgery attacks appli- cable to similar schemes in the literature. FROST improves upon the state of the art in Schnorr threshold signature protocols, as it can safely perform signing op- erations in a single round without limiting concurrency of signing operations, yet allows for true threshold signing, as only a threshold t out of n possible partici- pants are required for signing operations, such that t ≤ n. FROST can be used as either a two-round protocol, or optimized to a single-round signing protocol with a pre-processing stage. FROST achieves its efficiency improvements in part by allowing the protocol to abort in the presence of a misbehaving participant (who is then identified and excluded from future operations)—a reasonable model for practical deployment scenarios. We present proofs of security demonstrating that FROST is secure against chosen-message attacks assuming the discrete logarithm problem is hard and the adversary controls fewer participants than the threshold.