Choosing the right offshore custom software development partner involves evaluating several key factors. First, consider their technical expertise and whether their skills align with your project needs. Cultural fit and communication style are also important, as they influence collaboration. Review their project management practices and ensure they follow industry standards like Agile or Scrum. Additionally, consider their time zone and how it aligns with your working hours. Lastly, focus on transparency in pricing, timelines, and deliverables to avoid misunderstandings.