Amanda Bynes -- She's Mentally Unfit To Stand Trial

Amanda Bynes is mentally unfit to stand trial ... this according to her lawyer in her DUI case ... TMZ has learned.

Amanda was supposed to appear in court today for her DUI arrest in West Hollywood back in April, 2012. Amanda's lawyer, Rich Hutton, told the judge Amanda did not have the mental capacity to understand the nature of the legal proceedings.

The judge apparently had concern ... and transferred the matter to the mental health court. The judge in that court will conduct a hearing to determine Amanda's mental competency.

Meanwhile, Amanda is getting long-term treatment after being placed under a conservatorship.

Here's video ... of a noticeably unstable Amanda climbing into her Range Rover last year.