Did the New Testament writers use the Septuagint?

Yes — frequently. But the full picture is more interesting than "yes."

The New Testament authors wrote in Greek, and most of their readers knew Greek, not Hebrew. It made sense to quote the Greek translation of the Hebrew Scriptures that their audience already had. When Paul quotes the Old Testament in Romans, he usually follows the Septuagint (the LXX). When the author of Hebrews quotes Deuteronomy 32:43 at Hebrews 1:6 — "let all the angels of God worship him" — he is quoting a line that is not even in the standard Hebrew text. He is quoting the LXX, which translated a longer Hebrew original that the Qumran Deuteronomy manuscript 4Q44 (4QDeutᵍ) later confirmed was real.

So yes, the apostles used the LXX. But "used" does not mean "followed exclusively."

When you work through the twelve most contested Old Testament quotations in the New Testament — the passages where the Hebrew and the Greek say something meaningfully different — the count runs like this:

• Seven follow the LXX form. Isaiah 7:14 at Matthew 1:23 (παρθένος, parthenos, "virgin"). Deuteronomy 32:43 at Hebrews 1:6. Amos 9:12 at Acts 15:17 (τῶν ἀνθρώπων, "mankind," where the MT has "Edom"). Isaiah 42:4 at Matthew 12:21. Psalm 40:6 at Hebrews 10:5. Isaiah 53:11's "light" reading (confirmed by DSS). Isaiah 40:13 at Romans 11:34.
• Three follow the Hebrew. Zechariah 12:10 at John 19:37. Zechariah 12:10 again at Revelation 1:7. Hosea 11:1 at Matthew 2:15.
• One is mixed. Matthew is eclectic even within his own nativity narrative — he quotes LXX Isaiah in chapter 1 and MT-aligned Hosea in chapter 2.
• One is adapted beyond either. Hebrews 10:5 reads "a body you have prepared for me" where both the MT's "ears you have dug for me" (Ps. 40:6) and the closest LXX form's "ears you have prepared for me" use "ears," not "body." A second LXX Greek Psalter tradition exists with "body," and Hebrews follows that stream.

The case that decides everything is Zechariah 12:10. The Hebrew root דָּקַר (daqar, H1856) means "to pierce, to run through with a weapon." Every other place the LXX translates this Hebrew root — Numbers 25:8, 1 Chronicles 10:4, Isaiah 13:15 — it uses the standard Greek equivalent ἐκκεντέω (ekkenteo, G1574). But in Zechariah 12:10 alone, the LXX translator departed from that practice and used a verb meaning "they mocked." When John needs to cite Zechariah 12:10 at the cross, he does not follow the LXX Zechariah. He uses ἐκκεντέω (G1574), the word every other LXX translator used, and he translates the Hebrew himself. The pre-Christ Hebrew at this verse, preserved in 4Q80 (4QXIIᵉ), reads דקרו — the same root the MT carries and the same root John translates.

That single case breaks the "apostles always used the LXX" claim. They used it when it was accurate. When it was not, they went to the Hebrew.

"And again another Scripture says, 'They will look on him whom they have pierced.'" — John 19:37

The full study Which Old Testament? traces all twelve citations with the original-language data and shows exactly where each apostolic author made his textual choice.