Logs contain critical information about the quality of the rendered service on the Cloud and can be used as digital evidence. Hence, we argue that the critical nature of logs calls for immutability and verification mechanism without a presence of a single trusted party. In this paper, we propose such a mechanism by describing a blockchain-based log system, called Logchain, which can be integrated with existing private and public blockchain solutions. To validate the mechanism, we create Logchain as a Service (LCaaS) by integrating it with Ethereum public blockchain network. We show that the solution is scalable (being able to process 100 log files per second) and fast (being able to “seal” a log file in 23 seconds, on average).