The closest beach to Parthenon (Sithonia) is Paradise Beach, just 3.8 km away. Within 15 km there are 42 beaches; below are the 12 closest to Parthenon, ordered by distance. Photos, map, reviews.
3.8 km from Parthenon

4.1 km from Parthenon

5.5 km from Parthenon

5.5 km from Parthenon

6.5 km from Parthenon

6.7 km from Parthenon

6.9 km from Parthenon

7.2 km from Parthenon

7.2 km from Parthenon

7.3 km from Parthenon

7.4 km from Parthenon

7.5 km from Parthenon

Paradise Beach, just 3.8 km from Parthenon. The rest are listed below, ordered by distance.
Within 15 km of Parthenon there are 42 beaches, with 69 across Sithonia in total.
Based on user reviews: Karydi (★4.5), Portokali (Orange) (★4.3), Castello Beach (★3.8).
The beaches are spread out around Parthenon. A car makes exploring much easier — local buses and taxis are limited outside peak season.
Parthenon is on the Sithonia side of Halkidiki, in northern Greece. The nearest airport is Thessaloniki (SKG); from there a car is the most convenient way to reach the Sithonia villages.