There is another - That is, God. See John 5:36.

There is another - God the Father, who, by his Spirit in your prophets, described my person, office, and miracles. You read these scriptures, and you cannot help seeing that they testify of me: - no person ever did answer the description there given, but myself; and I answer to that description in the fullest sense of the word. See John 5:39.
And I know - Instead of οιδα, I know, οιδατε, ye know, is the reading of the Codex Bezae, Armenian, and two of the Itala. Ye believe the Scriptures to be of God, and that he cannot lie; and yet ye will not believe in me, though these Scriptures have so clearly foretold and described me! It is not one of the least evils attending unbelief, that it acts not only in opposition to God, but it also acts inconsistently with itself. It receives the Scriptures in bulk, and acknowledges them to have come through Divine inspiration; and yet believes no part separately. With it the whole is true, but no part is true! The very unreasonableness of this conduct shows the principle to have come from beneath, were there no other evidences against it.

There is another that beareth witness of me,.... Meaning not his Father, who is another, and a distinct person, from him, as the Spirit is another comforter; and both distinct testifiers from him, as well as of him. This is indeed the sense of some interpreters; but the Father is particularly mentioned in John 5:37; and the thread of the discourse, and the climax, or gradation, here used, show, that it is to be understood of "another man", as Nonnus paraphrases it; of John the Baptist, who is spoken of by name in the next verse, as a witness; and then a greater than he, the works of Christ, and then the Father:
and I know that the witness, which he witnesseth of me, is true; for John was now alive, though in prison, and continued to bear a testimony to Christ; and therefore he speaks of him as now bearing witness of him, and abiding by that which he had bore; and Christ knew not only that what he testified of him was true in itself, but that his testimony was a valid and authentic testimony, with the generality of the Jews; who held John to be a prophet, and looked upon him as a man of great probity and integrity, and whose word was to be taken: nor indeed could the sanhedrim, before whom Christ now was, object to his character, nor to him as a witness; nor ought they, since they themselves had so judged of him, as appears by their message to him, which Christ next fails not to take notice of.

There is another--that is, the Father, as is plain from the connection. How brightly the distinction of the Persons shines out here!
and I know that the witness, &c.--"This is the Son's testimony to the Father's truth (see John 7:28; John 8:26, John 8:55). It testifies to the full consciousness on the part of the Son, even in the days of His humiliation, of the righteousness of the Father" [ALFORD]. And thus He cheered His spirit under the cloud of human opposition which was already gathering over His head.

There is another that beareth witness of me. John is meant.

There is another - The Father, John 5:37, and I know that, even in your judgment, his testimony in beyond exception.