Jesus turned and said to PETER, “Get behind me, SATAN!"

[tonychanyt]

Was Jesus speaking to Peter or Satan?

The devil tempted Jesus in Matthew 4:

8Again, the devil took him to a very high mountain and showed him all the kingdoms of the world and their splendor. 9“All this I will give you,” he said, “if you will bow down and worship me.”
10Jesus said to him, “Away from me, Satan! For it is written: ‘Worship the Lord your God, and serve him only.’ ”
11 Then the devil left him, and angels came and attended him.

Jesus commanded Satan and he left.

A couple of years later, Jesus foretold his sufferings to the disciples in Matthew 16:

22 Peter took him aside and began to rebuke him. “Never, Lord!” he said. “This shall never happen to you!”
23Jesus turned and said to Peter, “Get behind me, Satan! You are a stumbling block to me;

Satan!
Σατανᾶ (Satana)
Noun - Vocative Masculine Singular

The Greek word is in the vocative case. Jesus addressed directly to Satan.

you do not have in mind the concerns of God, but merely human concerns.”
24Then Jesus said to his disciples, “Whoever wants to be my disciple must deny themselves and take up their cross and follow me.

The situation with Peter here is similar to Matthew 4 in that Jesus commanded Satan to leave. Satan obeyed and left as Jesus resumed his teaching to the disciples. Peter did not get behind Jesus. Peter also obeyed. He put away his human concerns. He resumed listening to Jesus, now without interference from Satan.

Did Jesus address Peter or Satan in Matthew 16:23?

Jesus addressed Satan when he said, “Get behind me, Satan!". Satan disappeared. Peter didn't.

Jesus addressed Peter when he said, "You are not setting your mind on the things of God, but on the things of man.” Peter obeyed immediately and did not talk back and focused on taking up his own cross and following Jesus.