The Type Ia supernova (SN Ia) 2013dy in NGC 7250 (d ≈ 13.7 Mpc) was discovered by the Lick Observatory Supernova Search. Combined with a prediscovery detection by the Italian Supernova Search Project, we are able to constrain the first-light time of SN 2013dy to be only 0.10 ± 0.05 days (2.4 ± 1.2 hr) before the first detection. This makes SN 2013dy the earliest known detection of an SN Ia. We infer an upper limit on the radius of the progenitor star of R 0 0.25 R , consistent with that of a white dwarf. The light curve exhibits a broken power law with exponents of 0.88 and then 1.80. A spectrum taken 1.63 days after first light reveals a C ii absorption line comparable in strength to Si ii. This is the strongest C ii feature ever detected in a normal SN Ia, suggesting that the progenitor star had significant unburned material. The C ii line in SN 2013dy weakens rapidly and is undetected in a spectrum 7 days later, indicating that C ii is detectable for only a very short time in some SNe Ia. SN 2013dy reached a B-band maximum of M B = −18.72 ± 0.03 mag ∼17.7 days after first light.