You’ll never guess which European country recently earned a credit-rating upgrade: Greece. Yes, that Greece. The eurozone’s perennial laggard suddenly finds itself six months into a remarkable economic turnaround.

Fitch on Friday upgraded Athens’ government debt one notch, to BB, and signaled more optimism about the economy and government finances. Athens remains two levels below investment grade in Fitch’s eyes, but investors are more bullish. The yield on Greek sovereign bonds fell to an all-time low of about 1.15% Monday,...