In Luke 17 the Pharisees asked Jesus when the kingdom of God would come.

Luk 17:20  Now when He was asked by the Pharisees when the kingdom of God would come, He answered them and said, “The kingdom of God does not come with observation;
Luk 17:21  nor will they say, ‘See here!’ or ‘See there!’ For indeed, the kingdom of God is within you.“

What did Jesus mean by “the kingdom of God is within you”? 

The phrase “is within you” is a poor translation.  A much better one is “in your midst” which several translations have such as the “English Standard Version” and the “Literal Translation of the Bible”.

Luk 17:21  nor will they say, ‘See here!’ or ‘See there!’ For indeed, the kingdom of God is within you [in your midst].”

Jesus Christ is the king of the kingdom of God.  By saying that the kingdom of God is in your midst, He is saying that the king of the kingdom of God is in your midst. 

Jesus said a similar thing in Luke 10:9-11.

Luk 10:9  And heal the sick there, and say to them, ‘The kingdom of God has come near to you.’
…
Luk 10:11  ‘The very dust of your city which clings to us we wipe off against you. Nevertheless know this, that the kingdom of God has come near you.’

Jesus had come upon them or come near to them. 

For more information of the kingdom of God, see “What is the kingdom of God?”