Compatibility
What Apple Watch is compatible with iPhone 6+ ?
Apple Watch Sport
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What Apple Watch is compatible with iPhone 6+ ?
Apple Watch Sport
Basically used, that has not been upgraded to watchOS 6, or ild unsold inventory.
If getting used make sure the seller has followed these instructions
https://support.apple.com/en-us/HT208855
otherwise, you will not be able to use it.
Or wait until you get a newer iPhone 6s, SE (1 or 2), 7, or newer.
Any watch that has watchOS5 or lower.
Compatibility