Amyloid self-assembly of alpha-synuclein (alpha Syn) is linked to the pathogenesis of Parkinson's disease (PD). Type 2 diabetes (T2D) has recently emerged as a risk factor for PD. Cross-interactions between their amyloidogenic proteins may act as molecular links. In fact, fibrils of islet amyloid polypeptide (IAPP) (T2D) can cross-seed alpha Syn amyloidogenesis and alpha Syn and IAPP colocalize in PD brains. Inhibition of both self- and IAPP-cross-seeded alpha Syn amyloidogenesis could thus interfere with PD pathogenesis. Here we show that macrocyclic peptides, designed to mimic IAPP self-/cross-interaction sites and previously found to inhibit amyloidogenesis of IAPP and/or Alzheimer's disease (AD) amyloid-beta peptide A beta 40(42), are nanomolar inhibitors of both self- and IAPP-cross-seeded amyloid self-assembly of alpha Syn. Anti-amyloid function is mediated by nanomolar affinity interactions with alpha Syn via three alpha Syn regions which are identified as key sites of both alpha Syn self-assembly and its cross-interactions with IAPP. We also show that the peptides block A beta 42-mediated cross-seeding of alpha Syn as well. Based on their broad spectrum anti-amyloid function and additional drug-like features, these peptides are leads for multifunctional anti-amyloid drugs in PD, T2D, AD, and their comorbidities, while the identified alpha Syn key segments are valuable targets for novel, multi-site targeting amyloid inhibitors in PD and related synucleinopathies.