Spider aciniform (or wrapping) silk is the toughest of the seven types of spider silks/glue due to a combination of high elasticity and strength. Like most spider silk proteins (spidroins), aciniform spidroin (AcSpl) has a large core repetitive domain flanked by relatively short N- and C-terminal nonrepetitive domains (the NTD and CTD, respectively). The major ampullate silk protein (MaSp) CTD has been shown to control protein solubility and fiber formation, but the aciniform CTD function remains unknown. Here, we compare fiber mechanical properties, solution-state structuring, and fibrous state secondary structural composition, and orientation relative to native aciniform silk for two AcSpl repeat units with or without fused AcSpl- and MaSp-derived CTDs alongside three AcSpl repeat units without a CTD. The native AcSpl CTD uniquely modulated fiber mechanical properties, relative to all other constructs, directly correlating to a native-like structural transformation and alignment.