|This article is written from the Real Life point of view|
- This article is about the theme from Samus Returns. For other uses, see Vs. Arachnus (Disambiguation).
Boss Arachnus plays during the boss fight against Arachnus in its room in Area 2. It opens with a stinger, accenting the surprise of Arachnus revealing itself from pretending to be an Item Sphere. In contrast with most boss themes in Samus Returns, this theme is more playful than threatening, although it still creates considerable tension with its dissonant electronic instruments. It can be heard here: 
Boss Arachnus may be based upon the original Metroid battle theme from Metroid II: Return of Samus, composed by Ryoji Yoshitomi. Most notably, the stinger at the start of the battle is nearly identical in both songs. The harmonic patterns in Boss Arachnus are also reminiscent of the melody of the Metroid battle theme. This may have been a deliberate reference by Daisuke Matsuoka, although this is not confirmed.
- The Arachnus did not have any battle music in Metroid II: Return of Samus. Instead, SR388 Underground continued to play during its battle.
- Surprisingly, Boss Arachnus is not a remix of VS. Marukara, Arachnus's boss theme from Metroid Fusion. Instead, VS. Marukara is remixed as Boss Manic Miner Bot and played during the fight against the Diggernaut.