The symbol $+$ for addition originated in commerce, along with the symbol $-$ for subtraction, where they were used by German merchants to distinguish overweight and underweight items.
These symbols first appeared in print in $1481$.
However, Regiomontanus was the first to use it in its current shape, in an unpublished manuscript from $1456$.
Previous instances of the symbol had appeared in print from $1417$, but the downward stroke was not quite vertical.