For ordinary sewing: yes, and it should be the same thread as the top. Different threads top and bobbin have different stretch and take tension differently, which produces puckered seams that people then chase with the tension dial.
The exception is machine embroidery. Embroidery bobbin thread is deliberately finer than the top thread. Dense embroidery lays down a great deal of stitching, and a full-weight bobbin thread builds a ridge on the back that pulls the fabric and causes puckering. Fine bobbin thread keeps the back flat.
A second exception: some decorative work. If you are using metallic or heavy topstitch thread on top, ordinary thread in the bobbin gives a more balanced stitch and avoids overloading the bobbin case.
What matters more than the thread choice is that the bobbin is wound evenly, on the correct class of bobbin for your machine, and through the bobbin winder tension guide. An unevenly wound bobbin causes tension faults regardless of what thread is on it.
Do not mix fibres between top and bobbin — cotton on top and polyester underneath behave differently under tension and the seam will not be balanced.
Questions people actually ask
Should the bobbin colour match?
On a balanced stitch it barely shows, so for construction it does not matter much. On topstitching and anything where the underside will be seen, match it.
Can I use up odd bobbins of leftover thread?
For basting and for seams that will never be seen or stressed, yes. Not for construction seams where you want a balanced, durable stitch.
Last reviewed 2026-08-11